Could make an interesting fanfiction or world making to put those somewhere and have a reason they don't show up or get explained in the movies or tv series. Like there is a bedrock with the flintstones somewhere on the same Continent.
Seriously though, I guess they use those terms since the audience would understand, instead of finding ways around it, like saying a long time, part of the day instead of hours. For songs they may need to rewrite songs to leave those terms out, though the end product would be better in my opinion, it would require more work.