In practice, people just sign what's in front of them, and indeed even if they want to read and understand everything, they can't without a fair amount of training -- and the existence of lots of other people who will just sign what's in front them means that it's hard to negotiate based on what you're signing even if you do read and understand it.
(Digression: I bought a house. The amount of paperwork that I signed without reading was probably around 100 pages. This is a crazy way to live.)
This feels like a problem that is getting amenable to a technological solution. It's pretty easy to imagine a digital assistant that could understand common contract phrasing and even do things like say to a person, "This offer includes a non-compete clause. Based on your demographics and the position, signing a non-compete clause will cost you approximately $X per year. Would you like to negotiate for +$X per year? Y/N" The problem, it seems to me, would be to get buy-in to the idea that such digital assistants should exist and that companies would have to accommodate such assistants.