Here's why:
* Collective nouns refer to a group of things considered as a single unit. "Raindrops" refers to multiple drops of water, but we think of them as a whole, a single entity.
* We can use singular verbs with collective nouns: "The raindrops were falling heavily."
Other examples of collective nouns:
* flock (of birds)
* team (of players)
* herd (of cattle)