alt Hacker NewsIt’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.
English still feels hard to explain when things sound natural and when they don't.
You can say "there are some vendors in the street" and it sounds normal. But if you were to say "they are some vendors" it sounds weird. It's pretty firmly more natural to say "they are vendors."