Because bugs happen all the time when programming - especially when you're dealing with something as massive as an OS (and an OS that has now existed for over 20 years). Your complaints show you have zero understanding of how software development works, or even what the point of the beta is.
I cannot stress this enough, because so many people seem to simply not understand this (or refuse to understand it?), betas will have bugs, full stop. Ideally those bugs will get fixed before the beta process is over, which is why betas exist in the first place! This bug has been fixed, it does not exist in the RC build, the beta process did its job. The vast, vast majority of Mac users will have no idea this bug ever existed.
Bemoaning about bugs in beta software is like bemoaning about getting wet while standing in the rain. If you're using beta software, you better damn well understand that there will be bugs - just like if your standing in the rain, you better damn well understand you're going to get wet.