Players are signing these extensions right now because of the financial security they provide. A lot of these players have families (either back home, if they're originally from elsewhere, here in the states, or both) who are depending on them, so signing long-term deals now is very reassuring. Robert/Moncada and others who signed monster bonuses right off the bat don't really face this issue because they already have more than enough money to provide for both themselves and others, so they have more incentive to just play and see how much their value can increase.
So to raBBit's point, the Sox are gonna want to play the service time game to guarantee 7 years of Robert since he's unlikely to agree to an extension.