You're focusing too much on stats and damage; the question at hand is the value of the stats as they contribute to damage.
If you have 10,000,000 strength and 0% chance to crit, how much value do you get from +1 str? Nearly nothing. How much value do you get from +1 crit? A heck of a lot.
If you have 10 strength and 90% chance to crit, how much value do you get from +1 str? A lot. +1 crit? Very little.
Optimizing stats on gear is about relative increase to your damage. If your class had 50 autocritting abilities and a baseline 80% chance to crit because SquareEnix says so, then how much value do you get from crit? Wouldn't you want to buff your base damage instead to take advantage of that crit?
Another way to look at it is that you get linear gains from +crit or +ss. Wouldn't you rather get superior geometric gains by multiplying your crit or ss?
The largest area coverage from 2 linear values is always a square, not a rectangle.
Yes, most normal stats in MMO mechanics have diminishing marginal utility. Or soft diminishing returns in terms of value.
Crit is a chronic offender because plebs love to see big numbers and categorically ignore the fact that if they already have a ton of it, they should invest elsewhere.
Edit: Check this out; I put a lot of tender loving care into it: http://imgur.com/GpD1fh4