It's a regulation issue. Things were similar in my country until the state decided to sell the fourth frequency range licence at a reduced cost (to offset the lead of the other three) and made routing cheaper. In exchange the new provider had to commit infrastructure investments. At about the same time, it was legally mandated that transferring phone numbers between providers be simple and cheap. The prices cratered pretty much instantly.

I think I'm on a 15€/100GB plan at the moment (plus a bunch a stuff like 25GB a month of international roaming).