General grumpiness about this particular design decision aside, it appears that since the T2 Macs, the main SSD was being used to store core system firmware. On T2 I think it's BridgeOS that sends the EFI image to the Intel side to boot it, and it does the firmware verification and loading.
This setup has a benefit; namely, it is now impossible to brick the system with a bad BIOS/EFI flash, and you'll never need to use a hardware SPI flasher to recover. Just like the iPhone, you can enter Boot ROM DFU mode, which can't easily be overwritten, and restore all of the firmware on the system that's writable.
This is great for restoring systems to a known good firmware state, including SSD, EFI, BridgeOS, RecoveryOS. On a normal system, there is plenty of non volatile storage that is difficult to recover or reset.
The primary argument against soldering the SSD, or really any other part, down like this tends to be that "Oh, now you can't just save money by upgrading it yourself|replacing it when it breaks". On the other side, these machines are engineered as appliances that should achieve a certain degree of reliability. The SSD is reasonably considered to last the "lifetime" of the product, and should it fail, I don't consider the inability to externally boot to be a huge problem, since whatever failure damaged the SSD is statistically likely to compromise the reliability of the rest of the board. Any failure is grounds for "rework" of the board on a hardware level if necessary,up to and including repair/replacement of the onboard flash, and not just a simple workaround in my opinion. Note that they don't force you to actually USE the internal SSD - you can still netboot or USB boot I think. It just has to work.
Trivia:
-At least on T2 systems, the flash memory itself (the NAND chips) on the mainboard can actually be swapped between units after a DFU restore by unsoldering them and transferring them.
The person that did this on YouTube notes that a common reason for SSD failure on T2 machines is due to SSD power regulator circuitry being located close to the intake vents on the side of the MacBook Pro. They recommend that users of these machines open them and clean the dust out often, since dust buildup on the actual power regulator combined with moisture sometimes causes a short circuit, sending high voltage to the SSD and damaging it.
This kind of failure (high voltage parts close to low voltage ones, close to liquid damage prone areas, without any underfilling) is documented by Louis Rossmann. It is, however, not always feasible or possible to entirely prevent.