Basically sockets add size, weight, and mechanical failure points (they can vibrate loose, you need an access panel, etc.). Since most people never use them over the life of the system it’s basically a bet that the RAM will fail less frequently than those other things, which is probably true but frustrating if you’re in the unlucky group.
It's a small sample size but I've seen more failed RAM chips than other components together. And a socket would make upgrades easier. Which of course isn't great if you're trying to sell higher spec up front instead of giving the option to upgrade later if needed. I'm not sure why a separate access panel would be needed, the whole bottom is easy to remove with a pentalobe bit.
Yeah, I’d be curious what the overall numbers look like for a large swath of users. I’ve never had a Mac I support fail do to RAM - hard drives back in the day, fans, GPU, or logic boards - but I did have multiple PC & Mac users who had reliability problems which were fixed by reseating DIMMs which had come slightly loose.
