But it does matter. ISPs simply don't have the means to provide capacity for every user to simultaneously max out their connection at once. That is not, and never has been how Internet infrastructure works. That'd be like building a 100 lane highway because on Christmas it gets congested.
You're paying for peak performance, not 24/7 performance.
You're paying for a rate, not an absolute amount. The internet infrastructure works on that rate (bandwidth). You don't buy a 10GB switch or router, you buy a 1gbps switch or router. ISPs peer based on bandwidth.
I do understand, though, that by having a data cap they're encouraging users to use less of that bandwidth. However, again, they're just passing on costs to the consumer instead of paying a fixed cost to upgrade their peering arrangements.
You're paying for peak performance, not 24/7 performance.