Cost of expandable memory
You add the cost of:


Connectors
The actuall RAM module that might be more expensive than the individual components
The fact that if you use a connector, someone will have to manually insert the modules, as I don't think this can be done by machine (in contrast to placing the components directly on the motherboard, as you would do when you don't use modules).


Finally, there is also the fact that you are increasing thickness - one of the main reasons laptop manufacturers are going for on-motherboard RAM. You can't get the thin formfactor of a few millimeters when you have a (in relative terms) thick fat connector for your ram modules.

There might be a few more reasons, but this is what I could come up with now.

I don't think there should be any impact on performance. The standards and specs are built to be able to go over connectors - they don't go any faster by ommiting them. On top of this the performance gain by increasing RAM speed is not very significant - that's what the registers, L1, L2, L3 ... caches are there for.