If a rebate is active for more than 90 days, it's probably the company's way of lowering the real price without changing the 'official' price. Sometimes changing the official price has legal consequences that they want to avoid. Manufacturers also realize that every sale doesn't have to be profitable. Sometimes they're willing to take a loss on one item in the hopes that you'll buy other things. The goal is to make you profitable as a customer.
Computer printers are a good example.
The manufacturer doesn't really need to make money on the printer. If it takes a rebate to get you to buy their model, that's fine. They're happy to make money on the ink cartridges that you'll be buying on a regular basis. They know that most consumers will spend more on the ink than they did on the printer. What the manufacturer gives up in product pricing they take back in shipping charges.