The first point is that LPDDR5 chip's operating frequency today has reached an all-time high of 9600 MT/s, and the latest off-the-shelf laptops (Intel Meteor Lake) are running at 7467 MT/s. This kind of high bandwidth is especially preferred in laptops for improved graphics and gaming performance. The iGPU has to compete with CPU for system DRAM bandwidth (which is already only a fraction of what a dedicated GPU has), every GB/s helps. Meanwhile, SO-DIMM is hitting its physical speed limit due to signal integrity issues (I heard that the impedance mismatch at the connector is too great to go faster). So yes, if you want high operating frequency, soldered RAM is the only option.
The second point is that, it's not always needed for every laptop, of course it's just a matter of tradeoff, many users would prefer lower DRAM bandwidth in exchange for better upgradability. On the other hand, hardware vendors can use this technical limitation as a justification to design machines with non-upgradable memory. You hit two birds with one stone: you can achieve better performance and force customers to pay extra for RAM at the same time.
If CAMM sees mass adoption, it's still up to the vendor to decide if they're willing to actually provide upgradability - but at least it'll be made possible at the technical level.