It’s not necessarily more complicated, but it has a strict reproducibility requirement. If I render TeX today, it has to end up 100% identical to what you get w

It’s not necessarily more complicated, but it has a strict reproducibility requirement. If I render TeX today, it has to end up 100% identical to what you get when you render it on your machine tomorrow, or what I get ten years from now. If a single tiny rounding difference makes a letter even a millionth of its width wider, that may mean line breaks move to different locations, that the number of lines in the text changes, or even that the number of pages in the text changes.