It requires a complement to complete sense, as:


He became ............ What?
She seems.............. How, what?
They are................. What?

There's a need for completion. These verbs don't make sense on their own unless you've mentioned before what they meant!

For example:

are you cold? No, I'm not. (I'm not cold)

you'd previously mentioned what you were talking about, so you respond to it by shortening the asnwer.