This is like asking why does Vin Diesel do action movies?
Why does Adam Sandler do comedies?
Why does Liam Neeson do thrillers?
It is what it is.
And yes, Johnny has much talent, so much talent that he can pull off OTT wardrobes. Can you imagine Tom Cruise as Jack Sparrow? No way. Or George Clooney as Sweeney Todd? Can't do it. Depp has found his niche for sure.
You compare Depp to Sandler and Diesel. Neither of those can act beyond cliches. Depp however showed he could do subtler roles in 'what's eating Gilbert Grape?' But that was one...And then, all he's known for has been multicolored OTT zilch. Diesel and Sandler can't act subtler emotional roles anyways, and Cruise can't act. When you think of Depp, you feel he has great talent, but you only see a tenth of it on screen.