Earlier quoted context omitted.
Because GPUs have had it soldered since the beginning, while desktops have had it slotted since (close to) the beginning. There are some exceptions to that rule, but it mostly holds. 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.
A longer time ago, sound cards used to have expandable RAM too. 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 :-)