Plans on service doesn't add up

With vodacom, I could change my plan with them, but I do not understand, is why are other service providers more cheaper than them, Like for example MTN, they have more or less similar plans, but MTN is more cheaper yhan half the price I am oaying right now with Vodacom, plus one gets more data on one's plan than vodacom, vodacom just wants money, looks like they are greedy to me.