Github, and virtually every other thing that costs more than $20 a month, targets primarily a B2B market. It might be popular with some local poor 20-somethings, but honestly, you're just an infection vector to get your day job on board.
The pricing is designed to extract maximum value out of business customers. If they have 125 simultaneous projects, they officially have More Money Than God. "The price of a residential Internet connection" is not a pricing anchor to them. (Should they need one, they're probably going to be persuaded by "We have 500 man-years of labor in our projects, one man-month costs us $15k, lemme break out Excel for a minute, oh it seems all my options cost pigeon poop.")
I strongly, strongly encourage you to listen to the Mixergy video about Wufoo or talk to anyone who runs a SaaS business if you do not understand where most of the money is likely getting made. That topmost plan which costs $$$$$ prints money, primarily from people who don't need all that it offers and couldn't care less because it costs less than pigeon poop on their scales.
If you don't use Github for your projects because $100 is a lot of money for you that's perfectly fine for Github because it does not make them meaningfully worse off.