I believe it's a limitation that is set by YouTube Itself, the maximum resolution which suits the Thumbnail is 1280x720,
If you wish to use anything beyond that resolution, it won't make much difference.
Furthermore, I do believe that you can definitely create 1920X1080 customized thumbnails using your Photo Editor/Thumbnail maker software/application but it may or may not work and moreover, you need to make sure that its size is less than 2MB
or else it won't let you upload in the first place. You can check the below screenshot as it is something that comes directly from YouTube blogs/Support blogs about thumbnails, I will leave the link below so you can use it for reference.
View attachment 12084
Reference - https://support.google.com/youtube/answer/72431
Being said this, I doubt this is something we can work on , as YouTube itself recommends their users to have a resolution of 1280x720 and since it's something default from their end and as TubeBuddy relies on YouTube API, we won't be able to work much on this.