They don't all have the same engines and some of what they do have is non-standard.
Meanwhile from a user perspective some browsers differ in extensions and configuration etc. So if you get a monoculture which causes sites to start relying on non-standard features of only specific browsers, it makes it harder for people to use or create other browsers that aren't bug-compatible with the only one anybody is targeting.
The "fast iteration" browser makers like is also damage, because it causes cruft to accumulate. Once you add a feature, sites start using it and you're stuck with it basically forever. See how long it took to get rid of Flash. But now they add features at such a pace that Chrome is now like 35 million lines of code, all of which is attack surface which causes all browsers to have disproportionately many security vulnerabilities.
Having multiple independent browsers requires new features to go through a standardization process that requires buy in from multiple implementations before sites can rely on them, which causes the changes to be fewer and more carefully considered -- a good thing for something you'll be stuck with ~forever.
If you think that, it's a testament to the skill of the web developers who have spent countless hours battling browser bugs and poorly or non-implemented standards to ensure that their site works and looks the same on every browser.
Not only do I believe that social media websites should be able to remove content arbitrarily, but that they ought to remove as much content as they can arbitrarily as possible.
this, combined with new government regulation such as EARN IT will make it as difficult and frustrating as possible to communicate on the web, and this will help people move from worse, censorship-prone forms of communication to more robust forms of communication on the internet.
