The agents all play this game of trying to wait for the other guys around them to sign first, so that they have a basis for their demands and won't end up looking bad in comparison. As a result, most of the 1st-round picks get signed in a last-minute rush. It reminds me of those 2-man bike racing heats where they go as slowly as humanly possible until a mad dash in the last half lap.
Another reason there's no rush is that the CBA, IIRC, allows drafted rookies to take part in pre-TC activities (i.e., the mini-camps) even if they're not signed, and they don't count against the roster limits until they do sign.
They do need to sign, however, before they can take part in the real TC.