Well, personally, I love birds because they're dinosaurs...
I'm only partly kidding! I'm one of those weirdos whose 'going to be a paleontologist' lasted from age 5 all the way into adulthood. I only switched majors and decided to become a writer halfway through my junior year of college. So yes, I do love that birds are dinosaurs, and that I can see a dinosaur almost every time I look outside.
But also my parents are birdwatchers, and infected me with the bug. I think that side of things is more likely to motivate the bird-photographers around here. As you said facetiously, birds know how to fill a niche -- they're so beautifully diverse! There are so many different strategies for success so well deployed, as well as so many bodyplans and colors -- all of that diversity is celebrated in birdwatching (and also laboriously notated).
And don't count out flight! Humans are fascinated by flight, as our heroes both mythic and comic book attest, so birds are compelling that way. And because of flight, they are often doing the business of animal life -- hunting, for instance -- out in our view, en plein air, visible for a long ways. Especially to the incredible lenses of the serious photographers who flock to Cake ;)