He frequently IS. VERY often in fact. Have you not watched any Warriors games this year, either?
They might be starting Bogut at center, but they switch into that smaller lineup with Green at the 5 very early in every game, and continue playing it quite many minutes every single game, if you start paying attention you'd notice that it's also the lineup they tend to have in at crunch time in close games. Why? Because it happens to the one that's been working best for them the last couple years.
Again, you really might consider trying actually watching some of these games you post about.
I have watched a lot of their games this year and there are only rare moments when they play without a center and it is typically when they are attempting to exploit certain matchups.