One of the technical reasons for not supporting passthru is that it would place many restrictions on the user that they wouldn’t like.
Passthru would require that start points for trimming must align with keyframes in the source video. For videos with really long GOP, those keyframes might be 10 seconds apart or more, which is severely limiting to the user’s artistic vision.
When it is time to encode, the codec parameters for any non-passthru material must precisely match the parameters used for the passthru material to avoid “quality strobing” in the final video. Strobing is when the quality level visually appears to jump from good to bad to good, which in this case would happen at the cut points. However, there is no way to reverse-engineer the codec parameters used by the source video. So matching the source quality would be extremely difficult. If the source video was encoded using a model of GPU that you don’t own, it could be impossible to match, as other GPUs and the software encoder will have a different look.
Also regarding codecs, passthru would require all media to be of the same format. It would not be possible to mix H.264 and AV1 videos on the same timeline because the codec cannot change format midstream in the output file. Neither could any media clip change frame rate or resolution or any other property. Likewise, a lot of music is MP3, but a common output format of camera audio is AAC, meaning music couldn’t be interleaved with the speech picked up by a camera mic since they aren’t the same codec.
The limitation above is further limited by the fact that no media can be manipulated at all. That means no fade outs, no filters, no speed ramping, and no mixing multiple audio clips, because that would generate new material that requires encoding instead of passthru. As described above, we can’t encode just the fade-out portion and overwrite the source in the middle of a GOP. The whole GOP would need to be encoded, which leads to those strobing issues again.
These are extreme limitations, to the point a user can’t even edit in what is supposed to be an editor.
Shotcut has an Extract Subclip feature that in essence provides passthru trimming. But for anything beyond this, passthru offers minimal benefit because Shotcut has encoding options for visually lossless and truly lossless output, which is basically as good as passthru but without any limitations during editing.