why do adcs think the map belongs to them the moment tower is down?

At 20 minutes the laning phase is over and unless the jungler is still actively farming and not grouping, the jungle is fair game. But what youre describing is "carry rotation" syndrome. The botlane tower is gone therefore the marksman rotates to the mid with the support following.

What do you want them to do if they get tower be on the opposite side of the map in trying to get the 2nd tier tower or come to your lane in which they won their lane, and instead get the next closest tower. Get the first set of towers in most cases solo in pre 15 if can't who ever won their lane or got tower, moves to the next lane and get their tower, 1st tier towers, move up, get the second tier towers, move up and get the nexus towers, move up get the nexus etc etc.
Other than MID, ADC players need resources and need farm constantly or they fall even further behind. If my ADC got their tower and is helping me in getting mine in which cause more pressure, sight and advantages I would be honestly happy because the game is inching closer to a W more than a L. That's just me tho