Earlier quoted context omitted.
A friend of mine is doing his PHD while being an intern at Meta. He does not share your excitement... at all. To summarize his complaints: a framework written a long while ago with design flaws that were cast in stone, that requires exorbitant effort to accomplish simple things (under the pretense of global integration that usually isn't needed, but even if was needed, would still not work).
How long has he been interning? Is it long enough for him to have learned how long the timescale big-tech roadmaps operate on? If he wants a feature, he better write it himself (if his PR doesn't conflict with an upcoming rewrite, coming "soon"), or lobby to get it slotted for the second quarter of 2026.
I'm not sure what your idea about PRs and features has to do with the above... he's not there to work on the internal infra framework. He's there for ML stuff. Unfortunately, the road to the later goes through the former, but he's not really a kind of programmer who'd deal with Facebook's infrastructure and plumbing.
The point is, it's inconvenient. Is it inconvenient because Facebook works on a five-year plan basis or whatever other reason they have for it doesn't really matter. It's just not good.
I also have no problems admitting that all big companies (two in total, one being Google) I worked for so far had bad internal tools. I don't imagine Facebook is anything special in this respect. I just don't feel like it's necessary to justify it in any way. It's just a fact of life: large companies have a tendency to produce bad internal tools (but small often have none whatsoever!) It's a water is wet kind of thing...