Steam offers easy game updates which has led to actively developed games that are private or public in nature. Steam can be set to download games while the player is sleeping but with fast internet this is not as big of problem like it was a decade ago.
Depending on the games installed, the update process can be daily. With many beta games the test process can see daily game updates which allow the testing to be a continual process.
Many of the friends on Steam are developers which afford an easy avenue for interaction. For the most part the developers are independent of the testers.
The gaming box has many high capacity hard disks so there is no pressure to remove games. Over time this has resulted in more and more games in the daily update queue.