Why not always use a stronger mix ratio for concrete?
Adding excess Portland cement will cause the concrete to shrink more and accelerate the hardening process making it even more brittle than it normally is.
My Dad used to always say there are 2 types of concrete. The kind that had cracks and the kind that will.
Cement really has no strength because it is brittle add something like sand and rock for it to bind together and it has some strength. Add rebar to further bond and it is even stronger.
The problem is over time it continues to hydrate making it more and more brittle. This process can take over 100 years to become a real strength issue with proper ratios used in the original mix.
Excessive cement can accelerate the process where failure is in a hand full of years compared to proper ratios.
Have you ever seen a bag of cement that got wet it is a solid “brick” but less than 10 years they were spalling and crumbling we had about a dozen bags that we used as riprap I noticed it crumbling at about 5 years and by 10 years it was like shale.
Using excess cement in your concrete will shorten the useful life of whatever you are making.