When it comes to dumb animals it's because falling in love requires a courtship ritual with multiple steps. If any of the partners doesn't follow the steps of the ritual then the other animal doesn't feel the emotions to go to the next step of the ritual.

If a male pig emits enough androsterone, a female pig goes directly into mating position.

When it comes to humans on the other hand it's possible to do fall in love quite randomly without it being bidirectional. Humans also don't have an a mating season and do mating quite randomly.

At the same time it's not so random that old impulses that make it more likely that love is bidirectional don't exist. Rapport between humans get's facilitated through mimikry where people who like each other copy movement of the other person. Breath syncs.

There are also things that happen on a deeper level and that are more difficult to explain. Sunday at the end of the Leipzig Solstice I had a conversation that was on a very deep emotional level. It wasn't love but still deep emotions. The woman I was talked to left the room because by empathizing with me about an issue that's for me painful, because she felt very strong pain in her heart.

Given my current understanding of emotional mimikry and emotions I consider bidirectional love at first sight to be quite plausible even if it doesn't happen to most people.

It also happens that people have sex before they are fully in love. Feeling pleasure during sex then get's associated with the sex partner and increases the feelings towards that partner.