it seems implausible that romantic love is often (usually?) bidirectional

I think you are correct on this one.

why do people usually fall in love with someone who is likewise in love with them?

I have a two-way answer. First: they don't. Unreciprocated love is quite a theme in Western culture, and I suspect in other cultures too, because as a situation is both extremely common and much dramatic. Second: there's also an active selection effect, where we tend to like people that like us and that we are more familiar with. Regarding social circles and romantic interests, it's an exploration / exploitation tradeoff, but the important thing is that there's an exploitation choice in the first place.