I think it's because some stability in association, "this character turns into this specific thing", was established and maintained early on in the most accessible or widespread fiction.
Those examples have the Cybertronian altmodes really made up and written in retroactively as a narrative device, to add weight to the "they're not from Earth" bit of the backstory. Meanwhile, the toys being sold were in the latter altmodes, which they'd spend the remainder of the show in to reflect that, the better to market those items. To the point where altmode changes were sort of tied to character changes as well (Megatron/Galvatron, Hot Rod/Rodimus). Notable as an anomaly is Skyfire, who's got fans of each look/ altmode as a result.
And they would never really change those as the show ran since the show didn't run long enough in the west to have something new (Powermaster Op, Goldbug, Pretender classics, Action Masters) to advertise.
Even in later shows like Beast Wars an altmode change tended to either need explaining or justifying, or got turned into a whole plot device. The movies had a lot of it occur offscreen, but we also have explicit situations of depicting a re-scan (Bee in 2007, Prime in AOE).
But yeah, myself I didn't really get too granular about it (this new Jazz is a curvy sedan but not a Porsche? That's fine). Although the Evac-body Universe Springer also never really read as "Springer" to me for whatever reason.
