「おえる」is paired with「おわる」。Somewhat contrived example:
Highschool has ended.
I have finished high school.
These two sentences are basically the same. Even if you use the active voice "finish", you don't have any control over when your highschool education ends. (Okay, you do have some control but not much.)
So, even though おえる is an active voice, its content is passive.
I have finished work for today.
Here you can finish your work quickly if you work hard. You have a lot of control. You can't finish your high school education as you finish your work for the day. So, すませる is a bit like "dispatch" in this context.
By the way, おわらせる is more similar to すませる than おえる is. In many cases, おわらせる and すませる are interchangeable.