If people could agree on what "look good" means, I don't think it'd be impossible to get the major browsers to update default styles. But every site seems to have a different idea of the look they want.
I'd be curious to see a somewhat-objective look at what's actually wrong with default browser styles, separating out well-established usability and design considerations from personal preferences and branding preferences. I wouldn't be at all surprised if there are near-universal things wrong with the default styles, and those might be possible to change if they can be separated out.
Based on my experience as a web developer working with many designers over some 20 years of design trends, here’s the problem:
Default styles aren’t pretty.
That’s it. They’re superior in every other way. You always know what to click, you always know what an element will do, how it behaves, the UX is consistent with your OS, etc. Default elements are fantastic.
Buttons in Chrome on macOS look completely different than in native apps.
Buttons and context menu in Chrome's native <video> controls look yet again different, following Material Design.
From the things one could argue are good about native browser styles "consistency with the OS" is definitely not one of them.
Depends on the element, the select and input elements are notoriously difficult to style, which is why people often remake them with divs and JS.
Combo boxes are a fucking nightmare to style, as are checkboxes and radios. Buttons arnt as bad, but you still spend a lot of time fighting against browser defaults which differ across browsers.
For some elements, yes. Many others have certain attributes that are completely unstylable, a lot of browser-specific attributes and selectors (not just prefixed attributes but also whole pseudoclasses), or both.
I'd be curious to see a somewhat-objective look at what's actually wrong with default browser styles, separating out well-established usability and design considerations from personal preferences and branding preferences. I wouldn't be at all surprised if there are near-universal things wrong with the default styles, and those might be possible to change if they can be separated out.