Why do you get Lucky Eggs as a level reward on level 40?

Because maybe down the line they will also increase level cap.

Brag post...? ;-)

I only say that because I have a LONG way to go...

Because you won't buy them anymore anyway

Simple. Because it would be strange to skip giving the player that item when the game gives you every other type of item (i.e ball, berry, potion, revive, lure, incense, incubator) upon reaching lv 40. Especially when you notice that they have been giving you all of the different types of item that's unlocked for the player every five levels since level 5.