Good luck finding a motherboard which will run at those speeds reliably. Sure, you can find a board which says it will do it, and it will sometimes manage those speeds, if you also have a CPU with a memory controller that can handle all the interference. When you solder the chips and thus make the traces much shorter and less susceptible to interference, that problem largely goes away.
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 :-)