Do you have an idea how much disk space the original copy (keeping in mind that it's always in the highest resolution available) takes? If the content creator has limited disk space (and many people do not have the money to keep buying external disks), it's not hard to imagine the original copy being removed to free enough space to create more content. Also, even if it was not deleted on purpose, there's a higher chance of the original copy of older videos getting lost (for instance, if it was stored in an external disk which was lost in a flood).

And there are also the videos where the "original copy" was on YouTube itself: unless you have a setup to also record locally (or mirror to another site like twitch), streaming videos are only recorded on YouTube.