Its because Tampa has been using him more as a blocker than a receiving TE... pretty much all of Gronk's injuries occurred when receiving the ball and getting his legs taken out, etc.. altho his original arm injury I guess was when he was blocking sergio brown on an extra point/FG attempt .. that was a freak injury
His knee being blown out by Ward was on a pass, his vertabral disc fracture was from taking a major shot on a pass, his ankle being bent into an L (high ankle sprain) was on a pass, his concussion was from a pass, his knee strain was from being hit low on a pass, his chest lung bruise was on a pass, his back vertebral disc hernia was on a pass, his 2018 concussion was on a pass
Gronk is a huge, lumbering target when he's out in space up the seam, etc... lots of DBs line up vicious shots on his knees/legs, even in the Rams SB he said he was up that night crying in pain because of the major quad contusion/bruise he got on a hit (from catching a pass)
Also seems like Arians is giving him and Brady the freedom to do their own training regiments, etc with TB12.. not sure if that has anything to do with it, but Gronk doesn't appear to have lost any strength
Could just be luck too, his body had an extra year to heal. but they aren't relying on him as the sole weapon/savior like the Pats were doing... he spends most of his time as in inline blocker, dominating defensive ends, LBs, etc