This summer (for another month and a half) United is giving members up to 5500 miles for a mid-size Hertz rental booked through the United website, 1 day or longer. Theoretically, if you had nothing else to do, you could do 1 day rentals over and over and over again, if you were living near an airport where rentals are very cheap, and run up a bunch of miles that way. But you'd still have to pay for all those rentals, and you'd still have to go to the airport Hertz location to pick and return each one. And it would only work at certain airports (some are way too expensive for that), and only if it was easy for you to keep getting to the rental facility every day or whenever.

Now that Manufactured Spending exists, fewer people do things the above to earn miles. But MS (Manufactured Spending) itself is somewhat harder for many people than it was in its heyday a few years ago.

Nothing easy / simple lasts very long. If it's too easy, the airline or credit card company or whoever figures that out and stops it.