Mostly because a poll is a consensus exercise. It's meaningless if my client displays a different result than your client. Of course, there are always polls that are just for fun. But the serious ones require additional tooling.
Discussion
Exactly!
I think the primary feature of Nostr is that this kind of consensus is not possible unless you agree to query the same relays.
All public access polls are just for fun. If you need to make a serious query you would need to declare ahead of time whose votes count and on which relay they must be placed.
The Fediverse does it like fiatjaf says, similar to Likes. However, the most accurate view of the poll will be on the poll author's home server.
On Nostr, we should a COUNT for votes on the poll author's relay.
When clients cast votes, they should deliver them to the poll author's relay.
This is a principle that clients should do in general for "p" tags.