You forgot the part that tanks and healers in general designed as if for bad/new/elderly players. Barely any mechanical requirements, lots of room for error.
The only support people willingly fill is Ana - she is also the only support hero with any mechanical difficulty to doing her job (healing), rest basically heal on auto-pilot.
Same with tanks - you will fill Zarya/Hog positions without issues, but Rein? That will be a lot of grudge.
Same actually true for DPS! Most players do not want easy-mode handheld gameplay. There is a reason most players do not want to even touch Mei, Junkrat or Torb (they have their fans, I know) even though these are much stronger than general consensus says about them. Devs for some weird reason cannot fathom this and keep powercreeping unpopular roles and unpopular heroes.
Aesthetics are important too, as you correctly mentioned. Tanks are lacking in this regard - the only cool/attractive tank in the game is D.Va and her follower base is huge. Other tanks are just comically goofy or ugly.
Aesthetic and lore is a reason, not all the reason. Gameplay design of course is also a big part of it. How strong a hero and how good it is to actually interact with said character of course are big part of it too.
There’s an anecdote that goes around in game design. A racing game made its way all the way through alpha testing. The cars all had different designs, aesthetic as well as gameplay. There were small cars, big cars, red, blue, and to each car various bars of stats informing the player of acceleration, top speed, stability, etc.
Playtester quickly reported some cars were too niche, some cars overtuned, others undertuned, others were too hard. Turns out the alpha was rigged and all the cars were actually the exact same stats, zero difference, all the feedback came just out of the aesthetic.
How something is communicated to the players is often more important than what actually is. Players will break though that sometimes, particularly at higher levels of play I didn’t say otherwise, but Overwatch is also a casual game, I would bet even players above average go for it freely and aren’t measuring methodically their actual value, they’re just taking in the feedback.