...I suspect that is the real reason.That, and by making the storage modular, the number one variable in customer ordering no longer requires different motherboard SKUs.
They already have to manufacture half-a-dozen different main logic boards for the various CPU/GPU/RAM RAM options, now if they have to make each of those in 5 different SSD versions you end up with a huge number of permutations.
That might be sustainable with the MacBooks and Minis which sell in fairly large quantities - but the volumes for the Studio are probably a lot lower, so some of the less popular permutations would have to be made in very small numbers (...and distributed internationally...?) - whereas with socketed SSDs the rare permutations can be assembled to order - or, at least, it delays the point during manufacturing where Apple have to commit to a SSD size.
I'm guessing there's some critical volume of sales below which socketed SSDs become more economical than soldered-in ones.
Meanwhile, ISTR that both with the 2019 Mac Pro and the 2017 iMac Pro, the RAM and SSD weren't officially upgradeable (even by Apple-certified engineers) at first, until they were.