[Ductapemaster]

Size, cost, and integration.

Size: SODIMM slots are huge. That space can be much more efficiently utilized when put onto the main circuit board. This potentially leaves more space for the battery, or making the laptop thinner/more aesthetic.

Cost: Supply chain optimizations drive down cost. The DDR chips they are using are likely similar to the ones they use in other hardware (think phones). This increases total volume, and decreases cost. Also chips aren't a finished good, so they are saving on the cost incurred in making RAM DIMMs in the first place.

Integration: They only have to make sure their hardware, firmware, and software works with the RAM they supply. Decreases the testing burden. No lookup tables to see what is compatible, and no guessing when you buy RAM that causes the BIOS to panic. It just works, and is highly optimized for the use case.