The browser is the only program on my computer that has a complexity that scares me.

I can look at the Linux source and figure out what is going on. There are some hard parts (synchronisation stuff and virtual memory is quite opaque on the first look) The code to handle layout and document processing in libreoffice is hairy, but I think I could manage.

A browser on the other hand. Layout and years of accumulated corner cases (handling the infinite variety of bad code out there) proper CSS rendering, multiple JITs, a shit-tonne of state and sandboxing of things that weren't meant to be sandboxed in the first place. Most, or maybe even all at the very bleeding edge of CS research.

effectively, browsers are growing up to gain most of the same functionality as modern operating systems, with all the complexity that brings.