Why is the recommended pressure for a bladder tank 2 psi less that well pump activation pressure
Because the pump does not start instantly. The 2-3 PSI below cut-in is to provide water while the pump is starting - without it, pressure will drop to zero (or very nearly) every time when water runs out but the pump is still spinning up. Indeed, you can evaluate actual adequacy of pressure (in case your tire gauge and water gauge might disagree) by observing behavior as water runs and the pump cycles on - if it does not dip below the tank setting, you can try adding a bit more air, until it does, and then you can let a bit out until it does not. But most gauges are adequately precise that this is rarely needed.
If you want the pump to start less frequently, add more pressure tank volume (either a single larger tank, or additional tanks) and consider running at a lower pressure (look at a tank chart for volume from 20-40 .vs. 30-50 or 40-60 - the tank holds more water at the lower pressure, due to physics.)
Of course, you appear to have 4 psi differential, not 2-3. You may also have a failed bladder/diaphragm if the pump is operating very frequently, but if you are measuring the tank pressure with the water pressure at 0 and a drain valve open, as you should, and it is holding at 28, then you should either add a pound or two, or adjust your pressure switch down a pound or two.
But if everything is otherwise correct and the pump starts after a gallon or two, you probably need more tank volume.