The more you play with a component's mechanical housing the more likely it is to break. There's also just human error in replacing hardware that leads to housing fractures.

Creepage among memory chips is also possible. RAM doesn't sit as snug for the entire life of your computer. It creeps out a little bit over time and allows for dust to prevent conductivity between the chip and the slot. Having everything soldered in place is much better.

The biggest issue, though, is all the added space required for the housing. The slots, connectors, clips, etc make the product bulkier. While that's fine for a desktop it's not ideal for a mobile device.