Do you think we should try to make them more like car rental
agencies?

Maybe.

My thought is that an airplane is a resource to the FBO, and they
should try to achieve the highest utilization that they can. Renters
who reserve but don't use the aircraft are using the resource without
paying for it, which produces higher costs for everyone else.

There may be no safe solution to the problem; obviously, you could not
take your trip, so it's not your fault.

Maybe only rent to instrument-rated pilots? Maybe not rent out for
large blocks that increase the likelihood of weather cancellations?
Maybe charge a small penalty anyway to make someone *think* before
they reserve?

Some people cancel only when there's good reason to do so, but others
reserve on a whim just in case they might want to fly.