The problem is that you have to be really careful about the laptop you buy if you intend to do that. A lot of laptops now have soldered, non-upgradable memory, as we’re hitting the bandwidth limits of what DIMMs can do, but the bulk of the industry hasn’t transitioned over to CAMM2 (the new removable standard) yet.

EDIT: Here’s a Reddit discussion from four years ago saying that even at that point, it was very difficult to find any laptop models that didn’t have at least partially-soldered memory:

https://old.reddit.com/r/laptops/comments/uzjmim/ultrabook_without_soldered_ram_do_they_even_exist/