'Work' doesn't seem to be connected to 'being' at all, but 'doctor' and 'dentist' are nouns and are connected quite naturally to 'being'. Could you produce another example sentence that addresses your concern more directlly? I see no reason why 'be' must use adjectives only.
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Anonymouswe use 'be' + adjectiveWe do, but we also use 'be' + noun.