Don’t ask why. From the Wikipedia: Birdhouses.
The Wikipedia’s birdhouse, or “nest box” entry, contains frighteningly little information about birdhouses. Basically, all it confirms is that they exist, and they are houses made for birds.
So I’ll go ahead and assume they were invented in the Bavarian Alps by that region’s 18th century middle-school shop teachers.
Birdhouses are currently most sought-after by the American old, and maintain some popularity among birds.
Moreover, birdhouses are one of the most presumptuous human inventions.
“Hey, bird. I know you and your feathered ancestors have been perfectly fine on your own since the late Jurassic, but I figured there’s no way you could build yourself a home as nice as this one. Eh? Eh? We people have a fine sense of aesthetics, don’t we? See, it looks just like my house! Now you can live like a real person!”
And furthermore, birdhouses are another indication of how stupid birds are. If someone you didn’t even know — especially from a species that totally dominates yours — just set up houses for you at random, would you move in without a whole lot of suspicion? It seems way more likely it would be a trap, or haunted or something.
But birds don’t think that way, even though humans eat bird eggs for breakfast. Birds are just all, “hey, this seems like a good enough place to set up camp. I mean, look at how cute the roof is!”
They’re lucky that, in this case, the humans responsible usually have good intentions. But man, birds really have a lot of growing up to do.