I scaled down a video, but the bit rate does not change
The scale filter has no effect on the encoder's bitrate control. 

Yes, a scaled down video should have a lower bitrate if it is encoded with the same encoder settings as the source. In your command, since no encoder parameters are explicitly set, ffmpeg defaults to encoder x264 with rate-control mode CRF with value 23. Apparently, this results in the same bitrate as the full-sized source. Set a higher value to reduce size.

ffmpeg -i input.mp4 -vf scale=2160:2160 -crf 26 output.mp4