The thing is that with state-of-the-art equipment (not considering antiquated nor cutting-edge equipment) the price/performance curve tends to be flat at the low-performance part and then starts to rise towards high-performance. Meaning that at certain capacity upgrading it is not cost effective any more. At this point, one can decide either to upgrade anyhow and handle the increased price/capacity in some way (eg. charging customers more for same service) or not to upgrade and keep consumption under control.
A bit rough example: if you eat say 2 Big Mac per day, each costing you a $, why doesn't McDonalds offer you eat-all-you-can for 60$ per month? This would make their income the same as it is today, right?
Because many users would over consume and/or start throwing food away (even more than they already do). Which means that they'd have much higher costs ...
The fact is that many of heavy-users of unlimited (or nearly unlimited) plans abuse the bandwidth they've given.
Another common misconception is that telcos still pocket absurdly high profits. They do fine, but their profits dropped a lot. A decade ago, most profits in mobile telecommunications went to telcos and telecom equipment manufacturers. Only small share went to VAS providers and handset manufacturers. Recently things turned upside down: most money goes to VAS (Google et co. through advertising money) and smart handset manufacturers (Apple, Google, Samsung, ...) while less money lands n pockets of telcos and consequently telecom equipment manufacturers. One can not make real money selling infrastructure and unlimited plans are final proof of it. And to add a nail to the coffin: there are only a few telecom-grade equipment manufacturers and the competition between them is not as fierce they want us to believe. It's much less fierce than in general IT industry meaning prices are not always falling with sale volume as one would expect.
In short: as telco industry slave (I'm working for them) I can tell you that things are not nearly similar as in general IT, so one can not make direct comparisons.