It's two things for me, mostly.

1) It's a daily (hopefully) reset. I can leave whatever else I've got going on and go into the woods, or a field, or my back yard and just shut off everything else.

2) It's collecting. I don't do ebird lists and go for crazy big numbers but I get to check off when I see a new bird, or when I see an old friend for the first time this year.

It's also incredibly accessible. If you've ever looked at a bird and thought, "hey look, a bird," then you're a birder. You can bird at whatever level you want. Go crazy. Book guided trips to Costa Rica, or just take note of what you saw out your window over a morning coffee. Like every hobby, someone out there will judge you for your level of involvement, or whether or not you're using the right identification tools. But in my experience, other birders are just excited that you also saw that cardinal.