Because if everybody uses more data there won't be more congestion? Sorry, that does not even qualify as wishful thinking. What caps do is force people who use the network more than others to contribute roughly proportionally more to its investment and maintenance.
If you really expect guaranteed uncontested bandwidth, anytime, anywhere, then your network could only ever sell you as much as you'd get if every one of their customers would gather in a single cell and fire up their radios at once. You would dream of modem speeds - and not be willing to pay even a cent for that tiny amount of service.
To make the analogy more easy to digest. Let's say you have a highway that's commonly overloaded. Instead of adding more lanes or limiting the number of cars per hour, you start limiting the distance that cars can go. How is that going to help? It will still get congested.
So ISPs should stop pretending already that caps are a network management tool - it's not. It's a user fleecing and anti-competitive tool they use to increase their profits.
It helps by making people think twice about doing unnecessary trips. Unlike driving, where people's patience for sitting in traffic is naturally limited, appetite for IP traffic is almost unbounded. Data caps prevent the equivalent of continuously running your toilet flush.
If they are worried about congestion, they have only two things they can do: 1. build their network up. 2. start reducing bandwidth of each connection, if they are too many of them.
Q.E.D. So these claims that caps are a network management tools are false. They don't even make congestion a more rare occurrence. If network can't cope with simultaneous usage, no amount of monthly caps will fix it. The only way to do it is to build it up.
What caps do, is forcing users to pay for more expensive plan with less caps. Or disadvantage competitors against ISP's own services that are excluded from those caps. It's a rip off scheme.