Mostly because the birds have different requirements. At it's simplest, the hole can be optimized that the bird will fit in the birdhouse but larger predators will not fit in. And the birds might have preferences on what kind of entrance they want. Like would you want a normal door to your house or a 5 meter high and 4 meters wide one?

Then the size of birdhouse, that also has practical reasons. If the birdhouse is too small, the eggs/baby birds can be too tight and can even be trampled to death. If the birdhouse is too large, the bird has to use extra energy to finish the birdhouse with moss and stuff, exhausting the bird for no proper reason.

And then the birds might have different requirements. Like the front wall needs to be rougher than normal so the chicklings can climb out. Or that the entrance wall needs to be minimum 5 centimeters thick so the bird accepts it.

A lot of reasons why there are different kinds of birdhouses :)