Streaming is very capital-intensive and requires significant tech investment and potentially code changes from developers. Being a marketplace operator is asset-light - you develop a minimal IP (the marketplace app) and, if the product is desired, marketplaces are viral, enabling rapid growth. A marketplace that's sufficiently large allows a small company (how many people are needed to build the Spotify apps?) capture revenues from a very big market (nearly every musician in the world, alive or dead). it's a way waaay better business model.

Also, Google tried streaming, NVidia tried streaming, both have had little success. Imagine going to your YC partner and saying "yeah, I know google and NV tried it and failed, but we'll succeed, because... it's indie games?"