In the long run, all costs are variable. Phone companies lack the bandwidth to provide all their customers unlimited data all the time. Most of them can’t even provide full speeds to their existing customers at peak times. If they gave everyone unlimited data they’d have to get more bandwidth, and they’d pass on every penny of the cost.

Data caps make congestion worse, because you are more likely to restrict where you use data and people are predictable. You'll no longer use bits everywhere because you care less, you'll use it where everyone else does.