I suspect *most* people want four, which is why four is what's provided.
Those who aren't satisfied with four are the ones you're talking about. Even though plenty of them are saying five instead of six.
Anyway. You ask why they keep mentioning six? I can't read minds, of course, but my guess is that it initially got mentioned because previous games had parties of six and some folks around here have a sort of boomer nostalgia thing going on, where they're married to years gone by.
Others probably latched on when they heard six and used the number themselves. If the number had been seven or eight, they probably would've been saying seven or eight.
I think that’s a stretch if I’m honest.
It’s 4 because that’s a part size larian are used to and I believe because D&D tabletop sees 4 as about the right number.
Baldur’s Gate however has obviously always been a 6 party member affair. You can call it nostalgia if you want, but it is the number the franchise is built around and the number most on this forum would prefer to see.
Some like myself have mused that 5 might be a compromise number, but to say most want 4 is imho wrong. 4 is the number Larian started with for reasons mentioned above, not because the community wanted it.