Soldering DRAMs to the board saves height and improves ruggedness. It can also improve memory power a tiny bit because you can dial down the signal drive strength and dial up the termination impedance since you don't have reflections at the socket connector. If you only have 1 rank, you can also improve memory latency by maybe a cycle by virtue of not having to accommodate parts at different distances from the memory controller. These additional benefits assume that Apple is one of the half-dozen or so companies with whom Intel is willing to open their memory controller documentation.

The counter-intuitive thing to me about using memory-down (at scale orders of magnitude smaller than Apple) is that it can end up being way more expensive over the lifetime of the product. Module vendors can constantly shop and swap between DRAM manufacturers in order to lower their costs and module competition is fierce.

RAM slots are big pieces of plastic and connector pins. Especially if you have two slots it just wouldn't fit in a laptop with that height.

I would guess it is due to a couple of factors:

RAM can now be tested in one package on the motherboard and people can't accidentally change it. In the long run it may open some abilities w.r.t using RAM for other things since you know its exact clocking.

The production is cheaper since the MB does not need an extra "install ram" step which is probably a human.

You can probably win some height space by a couple of millimeters.

You have more control over heating.