Undateable is an adjective used here as a noun.

However, poor and rich, in addition to being adjectives, are also mass nouns.

Being mass nouns, they are treated as singular parts of speech even when they are inherently plural.

Undateable is an adjective used here as a noun.

However, poor and rich, in addition to being adjectives, are also mass nouns.

As mass nouns, they are treated as singular parts of speech even when they are inherently plural.

As such, you normally say, 'the blues', 'the untouchables', but because 'poor' and 'rich' are also mass nouns, you say 'the poor', 'the rich'.