At a onesie position like tight end, where standard leagues require you to start just one player, there isn't much of a need to take one early.
It's the same reasoning that applies to waiting to take your quarterback, another onesie spot.
Sure, Rob Gronkowski is a freak of nature, but an early pick spent on a tight end is an early pick that's not spent on a running back or receiver, the lifeblood of any good fantasy roster.
It's particularly beneficial to wait on tight ends this season as the position is a little deeper than it's been in recent years.