With the default dot range of 25km a single-engined plane is a single pixel. You see it against the clear sky, or against ground if you happen to look somewhere near when it moves. I usually spot the single engine planes 12-14km away, but if they move further I can keep track of them insanely far.
I for one use 1680 horizontal resolution. At 90 degrees of FOV, thats only less than 19 pixels a degree. I cant bother to run the sinis through a calculator, but how big is an aircraft with 10m wingspan(a 190) seen dead ahead/behind 25km away?
More than 2 pixels?
I know I'm an exception but I have no trouble whatsoever spotting the dots, any of the game resolutions I've played with, on any of the screens. On my own home set-up and others'. And like I said before my eyesight is poor and I have that on paper.
I'm not going to insert the "get stronger glasses" joke. I think most people just havent developed the correct search pattern and methods yet. Practice makes you master. AFAIK not seeing anyone, friends nor foes, was a very common phenomenon in real life too, where only the natural talents would not need some time(and many never learned) to get rid of the "battle blindness".