Among other reasons, there’s this:
Imagine a group of raindrops falling at perfectly even spacing; a perfectly uniform gust of wind blosw sideways toward them - the ones in the middle of the formation are sheltered by those on the edge, so the pattern (which was a uniformly spaced grid) gets ‘squished’ on one side. it can’t be squished into a single large ball of water, because the surface tension isn’t sufficient to maintain it as it falls through the air.
So it’s marginally easier for raindrops to ‘clump’ together than it is for them to be spread apart, at least under some conditions.
This is probably only one of many effects; others might include a tendency for an existing raindrop falling through a saturated area of cloud to trigger the precipitation of more droplets. I think it’s a fair bet that chaos theory will be in there somewhere too.