according to newton's law force = mass * acceleration , and pressure = force/area , then why not M*A=P*A?

F=ma

p=F/S (I use S(S)pace)

F=Sp - could be, but

1) m is not dependant on how fat you're moving or what punches you into the seat. So you can't equate these formulas together.

2) acceleration is totally different than area.

You could technically equate them, but it shouldn't be written as ma = pa because then the a would be confused as being acceleration or area for both sides of the equation, which isn't the case since on the left it's acceleration and on the right it's area

Obviously using the same letter for two different things is not mathematically valid.

But if you used m * a = P * A

a = P * A / m

you have the acceleration of a bullet within a gun barrel.

Or the maximum acceleration of any unloaded piston acted upon by a gas pressure.

In a strict consideration of the units, it is correct. But realistically it doesn't make a lot of sense.

If you had a hydraulic piston accelerating something like a load of earth at the business end of one of those machines they have on building sites, then you could use P*Area = Mass*acceleration. But it doesn't happen very often.

You could. There is nothing wrong with saying ma = PA.