They don't "let" Netflix do it. It's netflix's right to do so and the movie studios tried to stop them
To be more specific, by the first sale doctrine, when Netflix buys a physical disk, they are legally free to do with it as they see fit. They can rent it, sell it, give it away, or throw it into a shredder, and there's nothing the studios can legally do to stop them. As long as the disk doesn't get copied, the studio's copyright is still being respected. This is well settled law, as much as the studios hate it. If they want to sell the disk to the public, they can't avoid selling it to Netflix, who can then legally rent it.
That doesn't mean the OPs basic point about economics is invalid. The studios clearly have different policies about different movies when it comes time to offer streaming rights. They know that a popular recent movie, or a still popular classic, will have much higher disk sales than an older or less popular one, so they deliberately avoid making it available for streaming to maximize their sales revenue. Netflix can still offer it on a physical disk, but given that they're paying full price for it, they have to offer physical disk rentals as a premium service. By the time the disks are ready for the remainder bin, the studio has pretty much exhausted its sales revenue stream, so they can make more money by offering it for streaming. It's about the same policy that they've followed with making movies available for TV; they only do it after making as much as they can selling physical copies.