Most newer streaming series are already in 4K. It would depend on the streaming service you are using though as many now have basic packages in 1080p and premium packages in 4K. I think all the Apple TV+ series are 4K though.
OP is referring to purchasable TV series.
The likes of Game Of Thrones have been available via the likes of Vudu/Fandango for quite some time now.
Prime Video also has 4K TV shows available to purchase, Star Trek Strange New Worlds is one such show.
Its bound to happen at Apple someday, but at the moment I can only think the reason we haven’t seen any as of yet is they can’t come to the same deal with the TV networks/distributors as they have with the movie studios over not paying a premium for a 4K version that cannot be downloaded.