A lot of pop singing is (or is presented as) more "here are different sides of the same character" than "here are different characters". As you say, the self-as-brand. It's not all of it, and it's not impossible to do that in acting ("Here is Keanu on a bus" "Here is Keanu in a doublet"), but I think one is more accepted in one medium that the other. Though I may be putting my own interpretation on "good at acting" there.