I had to look it up too. In this case it’s Open Broadcaster Software, but they seem to treat OBS more as a name than an acronym so the usage seems appropriate. https://obsproject.com/
Because that's the name of the software. Google will return it as the first result. It is technically an acronym, Open Broadcast Software, but all the marketing material refers to it as OBS.
I often feel the same, but I think OBS is the name here: OBS may be Open Broadcasting Studio, or Broadcaster Software, but it seems to mostly go by and be referred to as OBS…