This could be rephrased, "Why is kernel development so complicated?" or "Why is FPGA development so hard?"

In any given technical environment that has sufficient time to mature there will be more information to process, more choices, and more surface area. It's just the principle of intellectual entropy. Stuff tends to get larger and more complicated and specialized because it is.

If you want something simple, why not invent it? There's nothing stopping the next person from coming along and coming up with something that gets the same job done a lot easier than what we have now.

Of course, the first time you try to implement a tiny portion of a browser primitive like drawing a table you'll quickly see why things are complex.