Sadly, those accidents still happen because people disconnect the seat safety. I was almost victim when I rolled a small tractor with a mower. The mower almost got me when the tractor did not shut off. Fortunately, something hit the PTO switch and cut the mower before it got me. Such accidents also happen when operators fall asleep and fall off the tractor.
Sorry about your wife’s uncle. I have known a few too and those stories always hit me hard.
They disconnect it because a) any bouncing leads to shut-off at most inopportune times and b) they cannot ever leave the tractor just for a moment, such as to move an obstacle or something they forgot. Not good to keep turning off and on the engine.
It’s nice in theory, but as with most safety items, there is still a cost-benefit ratio to consider.