Isn’t that really a just a bandaid over the real problem though?

The fact the developers of hg went so far as to make that shows startup time is a real issue.

So why not fix the problem at the source?

Indeed, it seems like a perfectly good solution to me. I guess it's something about purity and not being the perfect solution. Wouldn't it be great if python was as fast as a C program that took many times longer to write? Yes, but that would probably be magic.

At a guess: They didn't hear about it (keeping your ears open is a cost not everyone wants to pay). They don't want to bother with setting it up. They don't want to bother with maintaining it (even if it's as simple as reinstall every time you get a new computer).