Young eyes, old eyes, and then there are people who it seems are blind as bats! There is quite a spectrum in eyes.
Then the amount of scopes not set up to those eyes. The start is to have the crosshair at least in sharp focus to the sky.
Lastly to have that crosshair lie flat on the target with both in focus.
To add difficulty add variable magnification. If that isn't enough it add loads of magnification range too.

All the above is a huge ask out of glass. Better scopes accomplish it without too much fuss and little perceivable distortion.

Then how that glass deals with light. Strong light and little light. One that is forgotten is cross light and shadow.

How forgiving is the eyebox is a big one.

Then there is parallax and how thats dealt with. The range where most of the work is going to be done.

Lastly, repeatability if target turrets are going to be used often. Quality counts here, though if zero is left on a setting for years it becomes less important so long as it stays set.

As can be seen there is a lot to design in, plenty to get right and plenty that can effect the final result. Some scopes just seem to get more right than others. Some makes have a reputation that rarely gives a bad result, others its far more hit and miss especially if they have a huge range they are marketing.
The test is in the field. American scopes tend to be good in bright light. European scopes better in the gloom. Asian scopes can be all over the place and rarely do bad varied light well.
And then there is price.