I suspect the reason the feature doesn’t exist. May be that Camtasia is not a pure video editor so to speak? It lacks dozens of features you find in most programs.
Camtasia could certainly benefit from it. My primary editor of choice is Adobe’s Premier Pro. Practically every clip I add to the timeline is pre-trimmed in the source monitor.
For those of you that don’t understand how this process works. You preselect an “In and Out” points of the video to be inserted to the timeline.
In the image below I’ve selected 28 seconds and 2 frames of a 3 minute and 21 second and 16 frame video as an example. Using Adobe’s Premier Pro.
You can play the video back in real time with audio to make your choices. Or scrub through with the timeline indicator. Then, Using the keyboard shortcuts, I and O, or manually. You select the in and out points. Then insert “Just that Portion” of the video to your timeline.
It’s a much simpler and streamlined way of working. Much neater and less cumbersome then placing 15 minutes of video on the timeline. Just to trim it down dramatically before moving forward.
A couple of years ago someone proposed this as an idea for Camtasia Windows and it received zero votes. I just found the thread when I went searching this morning. I voted it anyway. Considering it received no traction two years ago. I thought some clarification was justified.
if unfamiliar with using the Library, this Camtasia Studio 8 tutorial might be helpful ...
How To Use The Library To Build Out Large Projects From Multiple Small Projects 2017-04-01
another tutorial that mighty be helpful ...
How To Use The Library To Build Out And Produce Very Large Projects 2017-03-31
the demonstrated concepts work in Camtasia 9 and 2018 as well