I suspect they began soldering it because the cable connecting the disk drive had a lot of problems in older MacBooks. Soldering may have been an easy fix for that.
I hate the soldering, too. I'm just offering an explanation that may make some sense given the context.
What cable? As far as I can recall, everything not directly soldered since about 2013 was ssd mounted to motherboard using either M.2 or a similar proprietary connector. I suspect it was Cook-ish financial engineering and that trademark Apple hubris (that they finally got over to re-add ports in the most recent MBP).
I like your Ockham razor reasoning, but given the history of this company, it would be very difficult to defend this claim. Soldering the SSD and memory made the device unupgradeable (in contrast to pre-2013 models where I have currently have 2x2TB!).
And 2.5 inch drives can and often do have ports directly mounted on the laptop motherboard. If you don't want to use a cable, then you simply don't use a cable. Nothing else changes.
It's less the cable than the connector, so, similar trouble with nvme ports (and ram slots, for that matter). I've barely had exposure to supporting laptop hardware, and have seen "thing shimmied loose" or "contact corroded" as the root cause of problems more than once.
I hate the soldering, too. I'm just offering an explanation that may make some sense given the context.