Because there's a LOT of people out there who have slow mobile data or poor signal strength. I think assuming that everyone everywhere has extremely fast internet is a bad assumption. Also there's an enormous amount of bloat in package sizes now, I don't think this really should be as bad as it currently is.
I agree 100%, in the use case I mentioned we were looking to either prove or disprove product hypothesis with real users fast so efficient code wasn't a priority, but rather to confirm wether what we were about to develop was valid.
I don’t know what your caching settings are, but my users download the app exactly once, and then only again if I push any update (or their browser cache randomly decides to clear itself, don’t really have any control over that).
Aside from that, they use the app literally their entire workday, so spending 5 seconds loading the thing isn’t going to hurt anyone.