I have never seen her wear trousers. Why not use ‘she’ instead of ‘her’?

No. In English the pronoun in this position takes the object form "her", "him" "me" or "them".

This is used in all similar verbs of sensation like "hear" or "feel"

I heard him eat a crisp.

I felt them creep up behind me.

It is also used with an -ing verb:

I saw her playing tennis.

She must have seen me coming

On the other hand, with a "that" clause, the pronoun goes back to the subject form.

I saw that she was eating.

Even when "that" is omitted.

I saw she was eating.

So to compare these sentences

I saw her eating {a fact of what I saw}

I saw she was eating {I learn what she was doing}

In context:

Have you seen Jo? — Yes, I saw her eating with her friends.

Have you spoken to Jo? No. I saw she was eating with her friends, and I didn't want to embarrass her. I'll talk to her when she is alone.