Why doesn't AT&T and Verizon have unlimited data?

I have the iPhone 5s for sprint. Awesome phone but omg sprint is awful. Never get good signal anywhere, and whenever I go to someone's house that's in a wooded area I have no bars. Yet my friends who have Verizon and AT&T are getting perfect signal. 2 things are stopping me from switching: 1) I'm in a family plan, so if I switch I would pay my Own cell bill but I'm only 18 so I don't have too much money. 2) AT&T and Verizon for some reason don't have unlimited data? Why is that? That's the one thing I would love to know is why don't they have unlimited data? They used to I know that, but not anymore. And I don't know any many GB I would need a month for my phone. I'm on Wi-Fi a lot but when I'm out at work or wherever I use my phone for twitter, Instagram and maybe YouTube and the internet. That's it. I don't stream movies videos music etc. So could someone tell me why they don't have unlimited and how many GB I should get a month?

They probably don't have unlimited data plans because a lot of people are on their network. Now if you still really want to be on their networks and you want unlimited data, you need to get an alternative carrier that have that such as Boost Mobile and Virgin Mobile. An alternative carrier is a company that uses one of the big carriers network. There's this one carrier that I will be getting it is called Straight Talk. The plans are sold at walmart and you get unlimited talk, text, and data (3GB of 3/4G speeds). They use all carriers so you can bring your phone and set it up there. Hope this helps and pick for best answer!

They figure most people won't go over the 50Gb a month which is the cap on Verizon right now. Especially with so many Wi-Fi hot spots around.
However other than that it is supply & demand. If you had every one of their customers start demanding an unlimited data plan they would start to offer it. At a high premium of course. I feel this is the biggest reason they now offer unlimited talk & texting plans.