Most videos are seekable, where are you getting your videos from? You may be able to tweak the parameters of the video creator to make them more seekable.
My understanding is that Shotcut (and other editors and some vdeo players) uses I-frames when seeking. For an explanation of I-frames see here (GOP, B Frames and Codec Threads - What Do They Mean?)
If the video’s I-frames are sparsely scattered through the video (i.e. there is a very high ratio of B- and P-frames to I-frames, it makes it very difficult to seek on the video.
For such videos Shotcut warns you that the video is non-seekable, and gives you the option to create one that is, that you can edit. Basically it copies all the frames of the video to a new one that that has a much better GOP to make seeking possible.