First, most pizza apps are just that, an app. That app has access to your phones location data through GPS and some even use WiFi location services. Hence it can send a nearly exact position to where you are standing.
Contrast that to the standard phone network and systems ANI/ALI solution, which still does not (completely) support GPS coordinates at this point. In addition, while there were phases (phase I and phase II) of cell phone location compliance put into place by the federal government, most networks and phone companies lagged far behind in implementation of those standards. On top of that, city and county 911 dispatch centers (PSAP and secondary centers) also have to upgrade their phone and CAD integrations to support better location services.
As for why it isn't being disrupted. Simple, looooooooong sales cycles for an extremely limited market that is vigorously defended by the incumbents. Seriously, it isn't rare for a 2-3 year sales cycle for a lot of 911 components and systems. 18 months is about the normal when it involves critical systems with 12 months being probably the fastest you see anything change. Not to mention, the partners you need involved to make a solution work and be palatable to the 911 centers are the exact same companies who want to keep you out of their market, so it isn't easy. Not impossible, just not very probable without seriously deep pockets to support what would likely be a 3-5 year development to first sale. It makes selling to enterprises look like a fast process and cake walk.