You're thinking about reliability in a different way from Apple. By the time the SSD fails, it's not Apple's problem anymore, it's your problem. (Or, more likely, Louis Rossman's problem.) Conversely, an SSD that gets jostled out of a socket if the product gets bumped the wrong way (maybe because the courier kicked the package around) means time and money spent on in-warranty repairs.
Also, Apple never actually used M.2. All of their socketed SSDs are proprietary form factors, though there are cheap adapters you can buy that will let you use M.2 drives.
<i>an SSD that gets jostled out of a socket if the product gets bumped the wrong way</i>
You know the other side of m.2 is screwed in, right? Anything that manages to "jostle" a m.2 drive out of the socket has likely also destroyed the machine.
Also, Apple never actually used M.2. All of their socketed SSDs are proprietary form factors, though there are cheap adapters you can buy that will let you use M.2 drives.