Eh, it's the convenience. Your $200-$300 bike (talking super, super cheap prices) is as likely to get stolen as your Citibike.

You'd have to carry around a lock as well and even then, some part of your bike (ie.: a tire) might get stolen.

Dogs love to pee by wherever you park it outside and they're a bit of a hassle to bring into an apartment building (especially if you have to do stairs) - you're constantly worried about dinging walls (including in your room) and it also takes up a decent amount of space