The reason MVNOs are allowed to exist is to supplement the pricing strategies of the Big 4. MVNOs, by virtue of their lower visibility (less marketing, often no physical stores) attract only the truly “price conscious” consumer, while other types of consumers just walk into the nearest red, orange, pink, or yellow store and pay whatever they are told to pay.
Lots of people think they are price conscious, most are often not. They are brand conscious, and then compare prices among the brands they are willing to look at. Most consumers haven’t yet clued into just how much of a commodity cell service can be.
Cell carriers want to charge the price unconscious as much as they will bear, without diluting their brand by also selling value services to the price conscious consumer. The MVNOs are a filter, an alternative branding system, that allows the Big 4 to do this.
tl;dr: because if you’re a cheapskate, things simply cost less. if you are too lazy to put any effort into life, things cost more.