Just to be clear it is probably a good thing that YouTube re-encodes all videos. Videos are a highly complex format and decoders are prone to security vulnerabilities. By transcoding everything (in a controlled sandbox) YouTube takes most of this risk on and makes it highly unlikely that the resulting video that they serve to the general public is able to exploit any bugs in decoders.

Plus YouTube serves videos in a variety of formats and resolutions (and now different bitrates within a resolution). So even if they did try to preserve the original encoding where possible you wouldn’t get it most of the time because there is a better match for your device.