Earlier quoted context omitted.
Modern CPUs are an order of magnitude more complex and usually built on a smaller process, yet we have no trouble putting them on intermediary PCBs to spread out contacts and allow larger pins. I see no reason we couldn't make LGA sockets for DRAM chips in the same way.
The beauty and success of DIMM modules is that they are a simple slide-in to install technology. It's easy to operate, cheap, robust, and it is very space efficient. Imagine RAM modules in an LGA socket instead... how much space would it require? How robust would it be? What will be the price? I am quite sure all of these issues can be solved, but is it worth it? Let's be honest, users who are interested in having up…
I don't think this is the driver of these decisions per se, but it is undeniably a bonus for the manufacturer; non-upgradable devices become obsolete faster, necessitating new replacements.