Streaming games don't require a lot of storage space like many GBs of space. The game streaming app maybe as small as a few MBs in size. Some people with a very fast and unlimited bandwidth internet/data connection may use a smartphone with 16GB or less space to play streaming games which are streamed from an internet server hosted at Google, Sony, or Microsoft.
There maybe fewer mobile gamers who buy smartphones with more storage space if Game streaming to phones become more reliable, and fast.