Why can't "asleep" be classified as a verb? [closed]

In English, verbs are words that take certain endings and can appear in certain constructions.

Verbs like sleep, go, own, or be have special forms:

• sleep, sleeps, slept, sleeping
• go, goes, went, gone, going
• own, owns, owned, owning
• be, am, is, are, was, were, been, being

that don't occur for other predicates

• *She asleeps, *She aslept, *She has aslept, *She was asleeping.

Asleep is an adjective that's formed from the verb sleep. Adjectives can be predicates (they're called predicate adjectives in that case), but adjectives can't take the endings that verbs can, so they have to use an auxiliary verb be that can be inflected. The same is true of predicate noun phrases like a doctor or solid rock

• That man is asleep/tired/tiring/dead/here/purple.
• That building is a clinic/solid rock/a monstrosity.

So that's why asleep can't be a verb; it's already an adjective, and it can't work like a verb.