It’s simply that, in English, there is no indefinite plural article — or, equivalently, that the indefinite plural article is null. So ‘they are doctors’ is consistent with the behaviour of plural nouns elsewhere.
Contrast this with, for example, French, where the indefinite singular un(e) correspond to the indefinite plural des: La boulangerie vend des baguettes ‘the bakery sells baguettes’ (whereas English cannot say *‘a baguettes’). But as in Spanish, professions take no article either way: Il est médecin / Ils sont médecins. So all three languages are self-consistent, just in their own ways.
Contrast this with, for example, French, where the indefinite singular un(e) correspond to the indefinite plural des: La boulangerie vend des baguettes ‘the bakery sells baguettes’ (whereas English cannot say *‘a baguettes’). But as in Spanish, professions take no article either way: Il est médecin / Ils sont médecins. So all three languages are self-consistent, just in their own ways.