Because it is a budget plan. They are buying 2,000,000 SMS messages per month because 25 years ago with g1 and g2 protocols it made sense to sell them that way, and then trying to divide that between their 10,000 customers without running out. Similar for voice and data, they buy so much and divide between all customers.

Contracts could be sold a different way. However nobody wants to because the big companies want an option for budget minded people to get a little money out of them, while still giving them bad enough service that they all but the cheapest will run to the "big boys" with the unlimited plans. The budget companies need something they can sell that is cheaper than the "big boys", realistically they have exactly the same product and so somebody needs to introduce some artificial limit someplace to make it work.