Would is a Modal Auxiliary Verb, a group of auxiliary verbs with peculiar properties. One of them is that they don't have any tense, so the normal mode of an English auxiliary verb is not open to them. Normally, the first auxiliary in the [verb chain] inflects for tense:
- The model [has been being photographed] for hours now.
The auxiliaries do, be, have, and get all inflect for present and past tense, like all verbs in English:
- He is not here today. ~ He was not here yesterday.
- He doesn't need any money now. ~ He didn't need any money then.
- He hasn't realized it yet. ~ He hadn't realized it yet.
- He gets married tomorrow. ~ He got married yesterday.
But modal auxiliaries don't inflect for any tense, present or past:
- He must leave tomorrow, but not *He musted leave yesterday
- He should leave now, but not *He shoulded leave yesterday
Since there are only two tenses (present and past) in English and neither is marked on modals, they're considered to be tenseless. There is no "future tense" in English, no matter what you've been told in school, since all modals can refer to any time -- will is another modal auxiliary and behaves like the rest of them).
So you can say "Why would you do that?" about any event someone has done, or is doing right now, or is considering doing. It's all "would-be", and that's timeless.