The density of the water probably does have something to do with it, along with the amount of microbial life.
But speaking from "this side of the counter" - YES, saltwater hobbyists in general, tend to pay closer attention than freshwater hobbyists. Reefkeepers have more time and money invested, and as such, tend to take better care, and observe more.
There are some very dedicated and conscientious freshwater hobbyists - so this is not a slur toward them - that's just been my observation having dealt with many hobbyists over the years.
Another factor may also be that many freshwater tanks are more densely stocked than saltwater tanks. If you have 20 guppies (and counting - since they breed like rabbits), if one dies, it may not be noticed right away - until it floats up or gets sucked up onto the intake.
Jenn