If all rebates were instant then they wouldn't exist. It's an inconvenient way to get a larger discount, only made possible by the folks who forget to redeem them. There is no free lunch.
It depends on the company.
I know most of them do it like you mentioned, so they don't have to payout, but, there are still some places that make this as painless as possible, fill out form online, then after 6-8 weeks, they send to the rebate amount. (Have to wait that long because of returns).
I also had some instant rebates from Staples and places like that as well.