Because Premiere Pro doesn't have access to the same set of codecs as those other players. (Editing software needs to be stable and allowing any Tom, Dick or Harry system codec into the fold puts that at risk.)
MPEG Streamclip is from a previous software generation and hasn't been updated in a long time. There are a bucketload of posts in this sub-forum about it not working properly on Retina displays.
When you 'Export' you are always recompressing. Even if you set the export bit rate to match the source bit rate, you are still recompressing. You'll have to judge whether the image or audio quality has taken an unacceptable hit.
To avoid recompression you can use the 'Save As' command (as moongravy suggested). Sometimes MPEG Streamclip won't be able to complete this request, sometimes your preferred playback software won't play the resulting file properly. The reasons for this take a lot of explaining.
