They practice with the team and don't have a contract during the minicamps and OTAs but during training camp they hold out if they don't have a contract.

because they have to be signed to go to training camp

Because they are Money hungry idiots who are going by what their agent says, all the agent wants is the 10%

Its all about the money these days. Show a bit of flash and get the demand generated then protect yourself by delaying the minicamps. I say, fine them every day they miss so there is an incentive to report! Agents are playing way too big of a role with young kids coming out of college these days.

most of them want to seem like they are committed to the team and then hold out so they can get more money

Hey, this is business as usual. Obviously, they attend the camps to provide a good faith gesture. However, at some point you have to a contract and holding out is their only leverage.