You don't need a 1kg heatsink, the Peerless Assassin is 135g and cools a 7800X3D just fine. Ryzen 7 chips are incredibly efficient, you don't need watercooling until you get into the 7950X territory, or Intel's higher end offerings.Just about everything has liquid coolers now for some reason. Perhaps it's safer to ship a system with a little waterblock attacked than a massive HSF. I'm guessing if the system gets banged around, something bad could happen with a 1KG heatsink hanging off the motherboard.
They do it because liquid cooling AIOs are probably cheap for them to buy in bulk and saying it has liquid cooling means they can sell it for 30% more. These little AIOs aren't even very good, you often get better performance off a good air cooler than a below average 240mm AIO.