Actually, many towers (maybe even most now with how dense small cells need to be) are not actually owned by the carriers. They are owned by independent companies and leased to the carriers, or the carriers lease space on an existing pole or building to mount their panels. If you've ever looked at a cell tower and seem multiple levels of panels... that's almost surely not owned by a carrier. It's most likely owned by something like American Tower Corporation, Crown Castle, or SBA Communications, and they lease physical space on that site to various carriers and other users. Line-of-site microwave connections also make use of towers quite heavily to relay direct wireless connections.

A lot of churches also lease space on or even inside their steeples/stowers.

And then there's also the small cells that can get installed basically anywhere... and often get installed on things like light posts which are owned by local governments.