Even School Zones for kids only require that you slow down to 15 and there
are lot more cars on the road than boats in the water.

Cars have brakes...boats don't. Cars can stop without throwing their
transmissions into reverse and then continuing to travel forward until
reverse power overcomes forward momentum. The more forward momentum, the
longer it takes a boat to stop.

Cars don't have exposed propellers either...so if one should knock down
a child at 15 mph, it's not as likely to take off an arm, leg or head as
a boat going the same speed.

Manatee Zones require that you slow to 6mph+/-.

Which is slow enough, provided that you also keep a proper lookout so
you can see anything in the water, for a reverse throttle to stop the
boat pretty close to immediately...faster and the boat will continue to
move forward.

Are manatees more important than kids?

A manatee isn't the only thing you could hit. Learning a little bit
about the dynamics of what makes your boat move and what it takes to
stop it could also save a kid's life...at the very least, it could
prevent damage to your boat.