I work in the industry and can't remember seeing a site (of any carrier) without battery backup. Only 1 or 2 carriers in my area, NY metro, have fixed gensets at the sites. Mostly the old wireline carrier who built with gensets from the get-go. I prefer the gensets integrated into the cells personally.

There are 2 other issues here though that are outside the control of the carriers.

You can have all the backup power you need on site, but still go off the air 'cause your backhaul fiber from the LEC or whoever has defective/dead/nonesistant backup power. I have rolled a generator to a cell down for power only to have a dead battery in an underground fiber amp that I can't get at keep the cell down.

Even if you want to add a genset, considering that in many areas of the country the carriers are so insanely regulated that you need a building permit to replace a broken antenna, or even to climb a tower to investigate an issue, it could take 18 months and hearings, fees, etc. for each site you needed to upgrade. In the end after all this you can end up denied anyway for fear the genset may "make noise" if it's running. Also, any change to a lease with your landlord is not going to typically be free or easy. Many rooftop sites in the City of NY are limited to a fixed percentage of roof space, so without the understanding and cooperation of zoning/regilating AHJ you can't add one anyway.

It's not as simple as the cell carriers don't want to spend money (that's just a small part of it )To insure emergency coverage you really need everyone involved with the issue to be able to see the big, comprenhensive picture, realize the benefit, and work cooperatively to make it happen.