Modern web engines are basically mini operating systems. Long gone are the days where a web browser just needed to render basic HTML pages, handle some simple protocol actions, and render images.
To build something that supports all of the latest web standards, is secure, is always up to date, and on top of all that, is performant, requires a large group of very skilled devs working constantly on all those components.
Web development, for better or worse, has become a massive and rapidly evolving ecosystem that is constantly morphing and changing. Web apps are becoming the standard, and even "simple" modern websites are absolutely filled with different widgets and frameworks for all the different elements they contain.
If a very large/rich org or company decided to dedicate a whole team of devs to build a FOSS web engine, it could happen, but that used to be Mozilla, and look how that has slowly been failing.
What person with a website that has any significant traffic would willingly break it for 80+ percent of its users? That will never happen, sadly.
Two stories:
I work in IT. Most people are nice and reasonable, but every now and then, there are jerks.
For the most part, everybody gets equal treatment from me, but if you are a super polite and friendly person, I'll bend the rules for you. I've given a few people unauthorized hardware upgrades, boosted their ticket priority, helped them bypass company restrictions, etc. Little favors for being so chill and easy to work with.
But in the other side, a handful of folks have gotten my evil side. One guy in particular, a real douchebag. Super angry all the time, a jerk to me and other employees, was always spamming us angrily to fix his stuff. He would constantly lock himself out of his account because he would angrily type the wrong password over and over and then call us all pissed because he was locked out and couldn't get any work done.
One morning he did it again, called the help desk and I was the lucky one who picked up. He ranted at me about how he had an important meeting in less than an hour and his account was locked out again, (because he kept typing his password wrong like an idiot.) He swore at me and yelled about how the password policy was bullshit, blah blah.
I had enough and told him that, while I could reset his password, unfortunately we recently updated our servers and it would take roughly half an hour for the change to take place. He yelled about how he was going to miss his important meeting and all that, but I just kept gently apologizing and reminding him that I didn't come up with the password policy and all of it was above my pay grade.
He hung up furious and I smiled, made a mental note to reset his password in half an hour, and marked the ticket as resolved. Still don't feel bad about that.
Second story: In college, whenever there was paper due that I had procrastinated on, if it could be submitted to an online portal, I would create a fake Word document, fill it with random characters, and save it with the proper name.
Then, I would use a hex editor to corrupt the document, just enough so it would still get recognized as a legit Word doc, but if you tried to open it, Word would throw an error and not be able to open it.
Then I would submit that the night it was due, so it would look like I had submitted my paper on time. Even with small classes, it would usually be at least 2-3 days before the professor or TA would get to my paper, sometimes up to a week, and that whole time, I would be working on my real paper.
I would get a message or email from the professor a few days later letting me know that for some reason, my paper wouldn't open, and requesting that I resend it.
I would then respond with something like, "oh hmmm, that's weird, not sure what happened. Sure thing, I just uploaded it again, please let me know if that worked."
Of course, the second time I actually uploaded my real paper. Did that trick a half dozen times or so, never got caught lol.