By it's very nature, there is only so much they can do with WiFi. Any access point can only reliably handle a set number of devices, and a set amount of traffic at any time. And simply adding more access points isn't possible due to a limited amount of spectrum for the access points to use. They start to interfere with each other and it just gets yucky.
Think of the access point like a tour guide. A tour guide can only give a good tour to say a dozen people. And can only listen to and answer so many questions at a time. So the first dozen people, get one tour guide. Another dozen show up? Add a second tour guide. Now since they both visit the same stops, they have to stand far enough away from each other so they don't talk over each other, but close enough so that their groups can still see the stop. Now another dozen people show up, add a 3rd guide, and another dozen add a 4th. Except you now have 4 tour guides all standing at the train station trying to talk over each other to talk about the trains. And no one can hear their tour guide. And it just gets worse if you add a 5th, 6th, etc tour guide. Everyone just starts talking over each other, and no one gets a good tour.
WiFi access points are the same way. They all start talking over each other, devices stop dropping off the network, etc. You hit a point of diminishing returns. The same thing can happen if you are in a very dense apartment building.