You are not in the business of shipping a million+ devices.
* Any connector is a potential failure point. Generally with a connector you double the connection points (ie, SSD -> Connector -> now soldered to board). So if you are soldering anyways, and can solder to board directly, then you save yourself both a part and a bunch of connection points. This is good news for everything (signal loss, noise / RFI / durability etc). When you warranty and offer applecare on expensive devices, you want these warrenty claims to be low.
* Apple's mode has been to integrate / tighten things up - that is their design philosophy. They are going to integrate as many chips into one as they can (ie, you won't be even able to unsolder a T2 chip because there won't be one). There stuff, again, is meant to be used as an integrated whole, there is no mix / match here.
* They are going to solder memory as well down - you won't be able to replace that either. Then they are going to try and integrate memory into same package as CPU / GPU. I realize this makes folks mad, but it's what they do and it's been very successful for them. I think some of their LPDDR stuff only comes in a solder on format (and M1 is going towards even more integration - I actually like mix and match on my desktops so don't use macs so not following so closely but I know they were thinking of literally not even soldering but integrating fully).
* I saw first hand a number of claims BTW where replacement batteries had been put into macbooks and iphones as part of resales and people then went and complained to apple or about apple online when those batteries had problems. I think even when replaceable, they've added warnings in these cases. But it goes to show the quality in the mix and match department is hit and miss.
Frankly, it's probably 95%+ of their millions / billions of devices that won't get an upgrade. They've done same with airpods, no removable batteries. A ton of companies, again, or COPYING these designs.