Why is it that saltwater tanks can't seem to support the same volume of fish that freshwater tanks can support?

It always seems to me as if saltwater tanks have a vastly smaller number of fish. I read stocking guides and think 'but that can't be right, there's only a few fish in there!'

I'm good at telling the above/below/just right stocking levels of freshwater tanks, I just haven't got the hang of it yet with saltwater.

I think it's because saltwater fish are more fragile - they can't handle bad water quality. The more fish you have, the more waste there is.

Why are saltwater fish more fragile than freshwater fish? I'm not entirely sure but I think it's because the ocean is so big - much, much more vast than a lake or pond where many freshwater fish come from. So just like with aquariums, the larger the volume the more stable the water quality is. Saltwater fish just haven't evolved to deal with inconsistent conditions.