With most transport-category aircraft, even with a 5 km runway, gross weight close to ZFW, using maximum flaps for takeoff, performing an unpressurized, full-power takeoff, the maximum legal tailwind is limited to 10 knots (or 15 knots, depending on the manufacturer) -- why is this? Clearly in a case like this, the aircraft could take off just fine with a tailwind above the 10 or 15 knot limit, when it takes off with a tailwind at the limit under much less ideal circumstances.
It seems unreasonable that there should be a fixed aircraft limitation for the tailwind component, rather than a limit that varies with runway length, aircraft configuration, temperature, etc.. Yet hard tailwind limitations exist. Why?