Yes they can add more neighborhoods (for example Mt. Komorebi technically has 6 neighborhoods, as the base camps are their own neighborhoods). However, there is no point in adding more neighborhoods if their goal is keeping the total number of lots down. That will just result in less lots per neighborhood.
Asking why the number of lots affect performance (when only lots in the current neighborhood are loaded) is like asking why Sims culling and Max Sim Count are a thing (when only Sims in the current neighborhood are loaded). It's not about what you SEE. It's about what the game needs to track behind the scene. It's like asking why open neighborhoods are not a thing when all lots are already loaded. Again it's not about what you SEE. The other lots in the neighborhood are only loaded physically for rendering purposes; logically the other lots are not loaded at all (i.e. things like object state/stats, routing/pathing info, autonomy graph, etc. are not loaded at all and thus these lots do not function at all).
Moreover, all Sims data, Lot data, Object data, etc. are serialized and then saved to a single save file. When you load the game everything is then deserialized. Your loading time will significantly worsen as your save file gets bigger.
