In theory JPEG has a lossless mode (in the standard), but it's not supported by most applications (not even libjpeg) so it might as well not exist. I've certainly never come across a lossless JPEG file in the wild.

Filenames also of course try to indicate technical compatibility as to what applications can open them, which is why .mp3 and .ogg are different -- although these days, extensions like .mkv and .mp4 tell you nothing about what's in them, or whether your video player can play a specific file.

At the end of the day it's just trying to achieve a good balance. Obviously including the specific bitrate in a file extension goes too far.