Because as it is now Chrome (read: Google) can dictate how the web works and they do. If you use a Chromium based browser everything just works and happy people doesnt often complain or see the problems. So using something Chromium based makes you part of the problem. Apple isn't much better though so there aren't many options left...
What prevents forks of chromium from sharing amongst each other? I don't think google can gatekeep to the extent you seem to be implying. I'm pretty sure Brave and Opera and MS Edge can share changes they've made amongst each other without going through the original repository first.
Nothing stops them sharing but they aren't big enough (and in some cases not interested in) changing the web standards.
Think about it like in this oversimplified/stupid example: A "fix" or change is implemented in Chromium. In the long run (intended or otherwise) it turns out it moves pixels slightly differently than the way Firefox does it. If almost all your visitors use Chrome you have to design your site to be perfect in chrome and you might do so in Firefox. Now you have sites that look as intended in Chrome but maybe look as it should in Firefox. This make Firefox users use Chrome more.
Now Brave et. al. is part of the problem, helping drive the only real competition out of market (and killing their only way out should they some day need to change engine).
In extremely complex code this is very hard to not be a part of and a company like Brave is way too small to fork Chromium for long if at all.
You underestimate how much of effort and money it takes to maintain fork of a browser. Chromium is project as big as Linux kernel (if not larger), but it's primary developed by Google emplyees so they have huge influence on direction of the project.
Because it creates a web that is not meant to work following a standard but to work on only one engine.
What this means is that this engine becomes the de facto standard of the web and this standard is controlled by the main contributor of the engine.
Every browser is now constrained by Google's own decision about what should the web be. Sure, they could technically disagree by forking WebKit/Blink, but since websites are made to work with Blink, a disagreement means being incompatible with such websites.
Arguably the engine SHOULD be the standard. The W3C and WHATWG have never been able to reliably document, much less enforce, standards. Add in ECMAScript variances and all the other newfangled web APIs and it's a losing proposition. The standards bodies never could keep up with the pace of innovation. Might as well let the code BE the standard.
That's already the way it works in the real world... the standards are irrelevant and ignored, only caniuse and browserslist actually matter. Like it or not, Blink is the new IE6, and its marketshare is only increasing.
Ideally it would be something not controlled by Google but by an independent third party (hand Blink over to Mozilla, deprecate Gecko?), but good luck with that.
Maybe this system wouldn't be as ideologically pure as building compatible renderers to a set standard, but it would result in far better developer and end-user experiences as the web quickly standardizes to a single renderer. The world simply does not need 10 different ways to display HTML with 90% compatibility.
Yes but its giving too much power to one actor only. What if Google decides to stop supporting an architecture, or decides that it's ok to have 8GB ram as a requirement ? What if they choose to implement a hardware accelerated feature that works only with NVIDIA LATEST-WHATEVER-AI-VR-HYPE ? What if they want to implement "crypto payment" as a standard and, oh, that's GDollars ?
Of course those are just random made up ideas but the point I want to make is that it's giving only one actor the power to define what the future of our only and sole international knowledge network will be.
Yeah, that's why ideally the engine itself would be open-source (which it is, though largely controlled by Google). I wish it were further controlled by a third party, kinda like ICANN or Mozilla, but that also subjects it to political capture.
The thing is, the existence of Gecko never actually meaningfully challenged corporate oligarchies. Mozilla's mission was noble but they were never particularly effective at it... web standards went from IE6 being the defacto standard to the Wild West for a while to Webkit dominance to a Blink/Webkit duopoly. There was never a period where we actually saw a standards-based web ecosystem. It was always renderer-based. In that sense, I'd argue the Gecko contributors (and Mozilla as a whole) would have more influence over the web ecosystem if they abandoned Gecko and focused on the Chromium/Blink project instead, especially if they had override/veto power over questionable commits from any one corporation. As it is, Gecko/Firefox is less than 5% of the web. You can't influence, much less set, any real standards when you're just a rounding error.
Like it or not, Chromium IS the standard. Only when Mozilla realizes that will they actually have a chance to succeed at their mission, instead of being the beloved but always-losing underdog...
Google already does this. Ask OpenPOWER or *BSD users about the fact their patches must live outside the Chromium tree and be merged in manually, for example. Mozilla has been much more friendly to niche systems as long as they don't impact higher-priority tiers; OS/2 survived in tree for literal years because it was self-contained and non-obtrusive.
Because if Google decide to make a user hostile change to the engine (Chromium), because everyone consolidated on one engine, now there will be no alternative. The only recourse would be to maintain patchsets or forks, which could be increasingly infeasible from an architectural standpoint, especially from smaller authors.
One way or another, browsers are heading towards engine homogeny (or hegemony), but Firefox and Safari are at least slowing this process down to some extent.