At this point the web is about as complex as an operating system in terms of complexity. That needs really strong specific standards in order for it to work, and in turn projects like web browsers are huge and complex.

If someone wanted to build a web browser that only followed the simpler parts of the specifications, it wouldn’t work for many websites* and people would not use that browser.

*Whether or not sites need to be so complex is another question entirely, but the reality right now is that they are