I dunno, the tools are kind of there. Browsers have canvases and JavaScript and SVGs and sound. The communities are around; they're just kind of dispersed. There's no one website that is THE place for fun stuff. Instead, there are dozens, and most of them suck.
There's still fun stuff, though. I stumbled upon this bit of insanity just yesterday: https://tykenn.itch.io/trees-hate-you. It would have fit in fabulously with the old Flash sites.
Edit: looks like you linkes something created with Unity?
Not sure, I'm not versed in game dev. So maybe my point about creation tools is moot.
However, 3D content always seems very samey to me, in a way that cartoons and regular animation don't. So the rest of my comment should still express what I mean.
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Flash had a WYSIWYG editor aimed at media creators who treat programming at best as an afterthought.
Flash was mostly about ease of tweening and extremely flexible vector graphics engine combined with an intuitive creation tool.
So the "Flash vs HTML/JS/SVG/CSS..." debate is not just about technical capabilities of the medium.
Of course there are many fun web apps in the browser, or as native apps, too. But Flash attracted all kinds of slightly nerdy people with cultural things to say, not just web devs with a lot of free time.
What "HTML5"/browser web technology doesn't offer is this intuitive, visual creation pipeline, and this kind of speaks for itself!
Also, I think the Flash "creator's" age is not separable from its time: using Flash wasn't trivial either.
There were just more people with interesting ideas, free time, and a wholistic talent for expressing their humor and ideas, combined with the curiosity and skill to learn using Flash (of course only as a licensed copy purchased from Macromedia).
People like this today are probably more often hyper-optimizing social media creators, and/or not terminally online.
In other words: I don't think the typical Newgrounds creator would have taken the time and effort to translate a stickman collage, meme, or other idea into a web app / animation.
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And to add even more preaching: I think that "creating" things using AI produces exactly the opposite effect: feed it an original idea, and the result will be a regression to the mean.
It's not quite the same but it seems the people who used to be publishing flash games are now making indie games on Steam. With modern dev tools and engines it's possible for one person to make what used to be a team effort before.
The whole "friendslop" genre is what replaced flash games.
And there were some amazing RAD and prototyping tools in the 90s (mostly for DOS, but also for Windoze desktop apps.) You're right, we sort of gave up on the idea when everyone wanted to be seen as a "real" software engineer who knew how to sling Java on the back end.
and Pagemill and Sitemill. At Bell Canada we had a very early web dev team in '94-'95. At one point pagemill came out and we could hire mostly non technical designers to build web pages. At the time it seemed like magic. We didn't need to have someone who grokked vi standing next to a designer all the time. But the HTML pagemill spat out was horrid. It always added a space to the end of link text and never closed list item elements. I eventually wrote a command line tool that fixed pagemill's output because some of our other tools really didn't like the flavour of HTML-inspired slop it emitted. *
And then I moved to the bay area and noticed there was a road called Page Mill Rd. in Palo Alto and sort of laughed for a bit. Surprised Adobe didn't release a tool called Sandhill.
[*] to be fair, most WYSIWYG page builder tools of the era spat out some sort of crappy subset of HTML, so not trying to say pagemill was the only offender.