There is a lot of inertia behind it. Some companies will have thousands of ArcMap documents which need to be maintained.
And the ecosystem has value all on its own. No single product is the best. But taken together it is the least bad option. Particularly when you want the same team to be able to construct lots of fairly dis-similar pieces of technology.
Arcgis online works really well and is easily customized. I haven't been able to get qgis mobile to work with county shapefile data. Infact I can't even get the desktop version to create it's start icon script, so it's effectively unusable.
I have done a couple of large projects in agol and had some pretty serious issues.
* Vector layers are fast but you cannot easily query attributes for a shape within the esri web apps.
* Feature layers are dead slow for large datasets.
* The API sometimes relies on you submitting form encoded JSON documents which may or may not actually do what you want on the backend.
* API error message are not always descriptive.
* The API documentation is not great. For example try finding the correct string formatting for a date.
It is a mass of different systems clustered together into a tightly coupled mess.
Because it's much better than qgis. It's like Photoshop vs gimp. Technically there is nothing you can do in one that you can't do in the other, but the same can be said of MS Paint...
And the ecosystem has value all on its own. No single product is the best. But taken together it is the least bad option. Particularly when you want the same team to be able to construct lots of fairly dis-similar pieces of technology.