We are running a small experiment where 0.3% of viewers will see an auto-generated thumbnail, instead of your custom thumbnail. We are not removing the ability to create your custom thumbnail, but we hope to gain insights on auto-generated thumbnails for the future.
Because this is a productive use of your time. Not fixing demonetization issues. Not fixing the sub box issues. An experiment that no one asked for, and apparently no one knew about to begin with.