IME rolling up was (and still is) always done either at the table during (or before) play...
Dandy. Your experiences are very different than those of many people, myself included. People did and still do cook up characters in advance and in isolation for various reasons, not least of which being testing the chargen system out or making a pool of NPCs. There were also plenty of groups (especially in the 70s and early 80s) where showing up to a game as a new player without a ready character was very bad form because it was seen as wasting everyone's time. Same went for replacing fatalities if you lost a PC and the GM allowed you to start a new one mid-session - they weren't going to stop the game for you to make one, so having backups was pretty common in some circles.
And yes, you did see people show with absurdly implausible characters that they'd rolled up at home, "honest" - and sometimes they had, although they'd never mention it took them dozens or even hundreds of tries to do so. I knew one guy who was notorious for having 300 page spiral notebooks filled with potential PCs, one to a page, all made at home and as they died he'd rip out a page and go on to the next ASAP. Most people wouldn't play with him. Traveller might have been the worst for that sort of thing early on, by 1979 there was (crude) software floating around to auto-generate LBB characters and some people really did use it. All that's just one potential problem with making characters in isolation, albeit one rarely seen in 2024.
Why are you making your characters at home rather than at the game in the first place, is my question?
Because even today some groups see it as wasting valuable play time. Other times GMs will want finished character sheets before they even meet the first time because they plan to include character-specific hooks in teh game from before word one. Lots of folks do their chargen via email exchanges along with other session zero stuff even when they plan to play in person. Making characters at home is not the same as making them in isolation with modern tech, which is a big part of the argument for collaborating with your whole table when character building - and world building, for that matter.
Also some folks just get their jollies making characters whether they have an immediate use for them or not. Maybe they wind up as NPCs, maybe they find a PC role someday, maybe they just collect dust, for some folks the process is reward unto itself. And that's not some Ye Olden Days thing that died with the Reagan presidency, people do it even today, as proven by the countless "character build" discussions online - with many builds projected to absurd levels of advancement the character will probably never reach in play.