Manufacturing cost reduction.

It's a weird trade off to make on a business laptop, but everyone does it now, including Lenovo with ThinkPads.

If you want the upgradability and ports of a 2011 Thinkpad you need to buy a budget gaming laptop now.

I've read that socketed can be less performant than soldered but I don't have a good source to confirm or explain why.

If it’s a standard RAM interface then no it doesn’t make any difference to performance.