So you take performance figures from the POH, unfactored, representing what a test pilot could achieve while expertly following best-practice procedures in a mint condition aircraft. You add non-conservative corrections for the most easily estimated adverese conditions, such as a smooth, dry, short grass surface, temperature, and some head or tail wind. Now you have your best-possible, optimum takeoff or landing distance.
On top of that, you have the condition of the aircraft, pilot technique, pilot skill in accomplishing the chosen technique, grass that is a little longer and more wet, a strip that is not so smooth, slope that may be varying, possibly some gusts from the wrong direction, and so on and so forth... But do you add a safety margin for all this?
No! Because that would mean your flight couldn't take place, and such rules are "for babies" anyway. Let's just go fly.
"Gee, you know what? I see in the latest AAIB bulletins that there are an awful lot of fatal takeoff and landing accidents taking place... Why do you reckon that is?!
"
I don't think the safety factors are unfeasible. I think flying from insufficient airstrips is unfeasible. JMHO.