The words "what", "why", "how" and similar question words can be used in either plural or singular constructions. As people leaving comments have remarked, they are not the subjects of the sentences, nor indeed are they objects. Indeed they are mostly adverbs in such constructions, although they can also function as conjunctions, and in other ways. See Collins Dictionary on "why".
Some examples shoeing valid use in bothe singular and plural constructions:
- What is Jack going to do in college?
- What are the three of them going to do in college?
- Where is Jane going for her vacation?
- Where are Jane, Jack, and Susan going for their vacation?
- How are we supposed to finish the project, now that Fred has left?
- How am I supposed to finish the project, now that Fred has left?
- Who is that girl in the red shirt?
- Who are those girls in the red shirts?
- When am I expected to finish?
- When are we expected to finish?
- Whether he knows it or not, Jim must comply.
- Whether they know it or not, all of them must comply.
When the context does not make it clear if the subject is plural or not, it can be either. For example:
Where are you going tomorrow?
This could be asking about the plans of a single person, or of a group.