Reaper is only effective at point blank, any team worth its salt will never let reaper get anywhere near that close, hence his DPS is incredibly low overall
There is a reason Soldier/Junkrat/Torb/Sombra/Tracers top damage scoreboards, they can reach out and engage somebody quickly and constantly for high damage, and considering the tank dive meta ATM it's amazing more people don't take them to counter dives
Dives are not unstoppable, a Zenyatta with a Soldier/Sombra babysitter can rip the face off a Winston/D.Va in less than a second, the reason dives are the "Meta" is they try to dive a squishy target
Other ways to easily counter dives:
Torb + Orisa: Orisa Shields and Torb cleans up any Winston stupid enough to try to land right on the turret, Orisa's gun adding enough DPS's to melt anything that tries to go past
Junkrat + Zenyatta: Junkrats insane burst DPS and Zenyatta's discord will kill a dive comp before it can blink
Reaper + Sombra: Reapers ability to be a brawler at close range which is what his strength is along with Sombra's high DPS and hacking the D.VA every time she lands so she's unable to do anything nullifies the dive each and every time
Bastion + Torb: Torb engages at point blank circling the enemies and goes right at the D.Va, Bastion melting the Winston before he can blink while turret covers Bastions blind spots for any Tracers/Genjis
Reinhardt + Zarya: Reinhardt blocks against the Dive why Zarya steps out for a second to get a 100% charge and engages, when her bubble goes down she instantly bubbles Rein and they both engage, Dive is instantly dead.
Add in a single Pharah who does nothing by stays away from the encounter the entire match to shoot their backside at all times where they can't defend because they can't kill a flying target that far away because then D.Va and Winston would have to waste all their cooldowns just to engage her, thus taking away their ability to dive further, there is plenty of ways to kill dives as long as people work together
Oh, and Genji is only useful for their ult, which isn't saying much because a lot of classes in this game are only the most useful for their ult like D.Va and Mei