Adjustment of Status - I-485

I really do not know how to express my very frustrated mood. I went to I-485 interview last week and was informed to have additional review after the interview. And yesterday I got an email that they need to carry out my background check. I searched online a whole day trying to figure out what background check is. To my very frustrated, the information indicated that this background check process can be very slow and is like "black box", and it is very difficult to be tracked for the progress.

As far as I know, they should finish background check to schedule an interview. Why after interview do they still carry out background check?

Why am I the one need to go through this background check process? I know lots of people can get approval at the interview, which means they passed all the background check already?

Is anyone here experiencing the same case with me and can share his/her experience? Thank you so much!