On-board memory has several advantages over SODIMM's such as:
5.8mm thinner
Smaller Footprint
Better Heat Dissipation
Lower failure rate
Lower Price
On-board memory has a field failure rate of 0.03% over a 10 year period, so the advantages far outway the capability to replace it.
In the case of the LabTop Mk IV, using on-board memory allowed it to be small, thin and have dual-channel memory.