It does, that's how LaTeX starts up and why you have to run it as latex instead of tex. Ditto texinfo, and probably ConTeXt. For a variety of reasons the TeX engine still has to reload a lot of stuff because it might possibly have changed, which is a suboptimal situation; but there is a concept of a "dump" state that has most of the kernel initialized. I suspect people don't care enough to work around it, and the people most likely to do the work (TeX kernel hackers) are also the people who are most likely to be inconvenienced by it not noticing the tweaking they're doing.

Which is not to say it's not a good idea here.