The simplest: it's not the rain that makes the verification before booking last all summer.
Here in Canada, especially with our winters, we look for the sun, but often price and availability of leave take precedence.
A family from Montreal will find a good deal for Florida during the rainy season, and if they can all go, they will.
The cost and the possibility of traveling together outweigh the risk of a shower.
We take the umbrella, and save money, because rain is a minor constraint with low impact compared to other factors.