I'm not saying it was ever necessary for any of the computers (except maybe the passively-cooled MacBook Air), but that it was necessary for the phones, and so that design was carried over to the computers. There was a considerable gap (18 months) between the release of the M1 Mac Mini, the first Apple Silicon computer, and the release of the M1 Mac Studio, the first Apple Silicon computer with replaceable storage. I'm sure that time wasn't all spent on developing the storage socket, but much of it probably was spent on adapting to and utilizing the larger power and thermal budgets more generally.
There’s definitely a big difference in storage architecture since the storage controller is on the SoC, and likely for the reasons you mention.