For a long time, you couldn't really buy an unlocked phone straight from the manufacturer in the US. The closest thing was importing one is with the correct network bands (GSM), but that also kind of went away when VoLTE became a requirement.

Relatively recently, some manufacturers started offering unlocked devices sold straight from them, but it wasn't until Apple started doing it with the iPhone that it became a mass-appeal/well known thing.

So overall it's because:

• That's just how things were for a long time
• Carriers offer deals on their locked devices
• 2-year carrier contracts were a thing until the mid 2010s or so
• People are just used to their carrier being a one-stop-shop

Nowadays more people are aware about buying an unlocked phone instead of a carrier firmware/carrier locked device. But in the US I'd say most people only know about iPhones being offered unlocked.