Because this doesn’t fix things at all.
He should not be able to damage the Sentry from those platforms when the Sentry isn’t already under attack, period. Nobody should. (it’s fine if he shoots it from there if minions/players are already engaging the sentry though)
If his team is pressuring foes at the opposite corner, he’s still free to plink away at the Sentry with very little in the way of repercussions. And if players try to chase him off, they’ll just get shot at immediately by his allies.
There is another problem that this would create as well: You can’t run around the corner and dislodge him (or any character causing problems from that position for that matter) without exposing yourself to the Sentry for a lengthy period of time
Why not just make it so that the Sentry cannot be damaged if it hasn’t damaged a target for a few seconds? Another game I used to play (Awesomenauts) did the exact same thing with the turrets (equivalent of Sentry) and it completely eliminated the threat that long-range pokes posed against them.