There are one in three possible scenarios playing out.
1) Kraft and BB are taking the "lets see" approach with their 42 year old QB and letting it play out during the season. The NEP nuclear option is telling Tom to go find his value in FA and then the Pats resign him to a 2 year deal. There may be some element of "lets see how Tom holds up", but not enough to stop a deal. I don't think this is whats happening but I figure i'd throw it out there.
2) Its Tom who is waiting- not NE. Hes waiting to see how his body feels at the end of the year and is placating his wife who only wants him to play if he is healthy and not at risk for permanent cognitive or physical debilitations. For example, if Tom experiences a significant head or back injury, signing the extension now would bad for both parties (Tom gets the $$ but he needs to come back and he wants to play & end his career on his terms and the NEP loose cap space). I think this is legitimate possibility.
3) They have already agreed to the framework of a deal and need to wait until Aug, cut-downs or other deals are done (restructures, trades, etc) until Tom puts pen to paper. I put this as the most likely scenario.