Becasue "apps" generally are much dumber than a full OS app.
"Apps" live inside a closed platform with very limited input methods and do a specific task. Instead of using a multi-purpose tool to do many tasks, Apple's take on "apps" are that you need one specific app for each task. Instead of using one (1) browser to access http-content, I should download a large number of apps that does the exact same thing, only each of them are made specific for that exact content.
One app for reading New York Times, another app for reading FT.
This unification of the developer framework has strong incentives to quit doing proper tools and just relase stupid apps, even for a multi purpose machine like a desktop mac. That is dumbing down a great platform.