Gerund

The short, if not very satisfactory, answer is "Because we do".

We spend time doing things, not We spend time do/to do things.

AnonymousWhy is the gerund watching used in the following sentence?

Because spend takes a gerund-participial clause.

Other verbs license different kinds of clauses:

to-infinitival: We counted on them to support us. He seemed to like it.
bare infini

Anonymouswhy they use gerund (watching) ??? in the following sentences We spend time watching movies.

I wouldn't say that "watching" is a gerund in the We spend time watching movies. In my opinion, it's better to name it an ing-form as the "watching" is followed by its object "movies". The object, again, in my opinion, shifts the "watching

Anonymousit's better to name it an ing-form

“Gerund-participle”, a variant of “·ing form”, also works.