I've noticed that among audio codecs, Variable Bit Rate (VBR) is typically the default. In fact, Vorbis and AAC do not support a truly constant CBR mode, instead offering a 'constrained' or 'managed' mode.
On the other hand, The MP3 encoder LAME defaults to CBR 128 kbit/sec, and Opusenc has a "hard-cbr" option, which seems to be true CBR. I have also heard murmurings that "VBR is superior" but without an objective explaination.
Can anyone care to explain why CBR seems to be unavailable or not recommended in many codecs?