The job availability for many actors would become extremely limited if this were the policy.
There are (still) far more heterosexual roles than any other type; the vast majority are able bodied. The gay/lesbian/non-binary/fluid/trans etc and disabled would be offered fewer roles.
If a book were to be turned in to a tv series or film the director might have a firm idea of who he wants to the play the lead role. If that character is Welsh but the actor is English, he would simply change the character to an English man.
Even my Asian friends think it derisory to see Asian characters in tv series that really shouldn't be there, usually hovering in the background drinking a cup of tea, the ladies in saris. Sometimes playing a doctor with one or two lines. Best actor for the role - that's the only factor that should be taken in to account.