Videos from my Photos app don't appear in iMovie

The "optimize Mac storage" setting could be the problem. Your videos are stored in the cloud, with only thumbnails in your Photos library, except for the ones that you have recently accessed. When you edit a video you are accessing it. So I am speculating that that those videos that you have recently edited and thus downloaded are accessible to iMovie but the ones that are in the cloud are not. That might explain your initial observation that only some of your videos are available in iMovie but the great majority are not.

"Optimize Mac storage" is an option that you can choose or not choose. If you have sufficient space to store the Photos library content other than in the cloud, then you could choose not to use Optimize Mac Storage and instead select "Download Originals to this Mac". If you disable the Optimize feature, however, you would need to have enough space, and enough time to wait for all the downloading to take place. 1400 videos could take up considerable space and considerable time to download. Also, since the media would no longer be in the cloud, it would not sync to other devices and any synced videos would be deleted from those devices.

Have you considered putting your Photos library onto an external drive, with lots of storage space, that is formatted Mac OS Extended (Journaled)? Then you wouldn't have the necessity of using Optimize Mac Storage. No, syncing, though.

Since you have found a workaround you might want to leave well enough alone. Your call, of course. Also, you can drag your videos directly into iMovie without accessing through iMovie.

-- Rich

If the videos are stored in the cloud iMovie might not recognize them until you download the video to the Photos library.

Make sure that the box in the Photos app general preferences that says "Use as system Photo library" is grayed out. That means that it is already your system photo library, which is what you want in order for iMovie to recognize your photos.

Check you Mac systems preferences under Security & Privacy/Privacy and click on the Photos item in the left side bar, and then scroll to iMovie in the right hand side bar. Or you might need to scroll to Photos. Confirm that Photos is enabled for iMovie.

With earlier operating systems you may need to click on Automation in the left side bar instead of Photos. Try both.

If the videos are DRM protected, they would not be accessible in iMovie.

If you have confirmed all of the above, and still having no success, try deleting preferences. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project. Now see if iMovie recognizes your Photos app videos.

-- Rich