That's essentially what the Sega Channel adapter was. The service didn't rely on something like you'd expect from a modern cable modem. The games were broadcast in a round-robin fashion (presumably broken up into blocks, as I remember the time to initiate a download was never super long, but the whole process to play a game did take a small amount of time). The adapter thus needed hardware and software in order to decode with the one-way signal to download the menu and game data.

The main cartridge (with the cable modem) was presumably heavily subsidized by the expected recurring revenue, which relies on the ephemeralness of the games. Offering RAM carts (even at cost) would threaten that revenue as people can stock up on games and cancel their subscription once they've built up their collection.