[usrusr]

Because then you can only sell to countries that have 110V mains. All three of them.

If your product is more complex than a light bulb, dealing with mains power can be quite a hassle. It requires bigger and more expensive parts than negotiating for 20V DC with a USB PD source.

You might want to take a look at USB-C to barrel connector adapters intended to run older laptops on modern chargers. They are tiny and cheap, I use one to liberate a thunderbolt 4 hub from the massive power brick it came with.