This doesn't explain why non-soldered RAM isn't okay for graphics cards when it is still considered perfectly acceptable for desktop computers.
Not to mention that the bar of acceptibility is moving. We had RAM in slots in laptops for a long while, but it's (not so) slowly going away. You can still find laptops with SO-DIMMs, but it's not a given anymore.
Go back far enough (486) and cache was off-chip on simms too!
They usally came in 8 to 16 discrete chips with pins on both of their sides, to be soldered on board, or plugged into sockets.
Plus one or two 'tag-rams' as chips of the same size.
All depending on the chipset, its cache-controller, and which capabilities the board-maker decided to use/expose/support. Which sometimes the BIOS would get wrong, still :-)