Plus, one of the key features of Pokemon is to keep the Pokemon you catch through each generation. If you catch a pokemon in Leaf Green, you can trade it to the newest generation Scarlet and Violet. And considering how old this series is, some pokemon are nearly 30...
Old granny Smith will not know this, and may buy her grandson a pokemon game not knowing any better. And little Timmy may not know himself that Pokemon Radical Red isn't an official game.
You see the issue here?
Flash carts are also cheap. And roms are easily accessible. There are a lot of fakes in the GBA and DS markets because flash carts are cheap. Hell, I've managed to get 2 fake copies of leaf green. One from eBay, and another from GameStop. I lost them in some of my moves, so they are back out there somewhere, unfortunately.
And I do love these fan games, especially unbound. I really do recommend it. If you want a more mature/black-white style game, unbound is up your ally. Just remember that it's free.