Yes, but no one is buying 1 cent NFTS on OpenSea, you are comparing two completely different worlds, and OpenSea is just a third-party to sell them. OpenSea is more comparable to Amazon charging a fee for people to sell their products on their platform. It comes with publicity, visibility. Third party markets in splinterlands don't offer extra visibility to the game, they exist because the game exists, they are used because the game gave them visibility. People won't find and use peakmonsters without knowing Splinterlands exists beforehand, while people will find and use OpenSea before certain NFT projects are listed there.
OpeanSea pulls billions of volume because it is the GO TO for any NFT investor/enthusiastic.