Tony Gonzalez was taken 13th overall in 1997 by KC.. he did alright..
Vernon Davis was taken 6th overall by the 49ers in 2006 .. he also had a nice career.. Not as good as Gonzalez but 583 catches for 7562 yards and 63 TDs aint a bad career. That's 11 fewer TDs on SIX HUNDRED fewer receptions than Witten.
Tights ends, like most positions, maybe more, need a good team around them to succeed. If one is drafted highly he's likely going to a bad team and it's not a position that can easily turn a team around. So they end up sucking on a team that sucks.. Meanwhile tight ends drafted later onto good teams like Jimmy Graham, Gronk et al have great careers because they go to good teams with good QBs and other good weapons ..