If game has a big community and a lot of players it is more simple to sell in-game NFTs inside ecosystem of the project.
Very often NFT is possible to buy and sell only on the platforms marketplace. They do not sell NFT on other marketplaces.
That is why the liquidity of the ingame NFTs are not very high.
The more popular becomes NFT the more people use them. Now NFT marketplaces are popular and a lot of new NFT marketplaces appear.
Also many famous persons creates their own NFT collections to earn money and attract attention of the community.