The reason the sticks aren't larger seems to be because they had trouble scaling down the DRAM memory cells[1] since about 2016.
DRAM works, as you might know, by letting a capacitor hold a charge (or not).
This ability is a function of the geometry of the capacitor: the area of the anode and cathode, as well as the distance between them. Leakage current discharges the capacitor, hence why DRAM needs to be periodically refreshed, the industry settled on 64ms retention time per cell[2]. Leakage current tends to go up when the features shrink, due to there being less insulation between the conductors.
As mentioned in [1] it seems things are about to change, so perhaps we'll see much larger sticks in the near future.
If you were thinking of slots, I think that's more economics and feasibility. The Threadripper motherboards had 8 slots[3], since the CPU supported it, but they were quite expensive compared to regular Ryzen motherboards. It also requires a lot more power, so forget having that in laptops. And the slots take up space, so was only for the full ATX size[4].
[1]: https://www.allaboutcircuits.com/news/micron-unveils-1a-dram...
[2]: https://www.csl.cornell.edu/~martinez/doc/isca13-mukundan.pd...