Also, it is difficult to determine with much precision what surface conditions actually are. Every runway is different, the distribution of the contaminant, braking action, etc. is varied so it’s sensible not to start off with eroded margins.
You might get away with full power in a situation where reduced thrust was not enough, in retrospect. Why risk it? We can’t quantify contaminated runways to the same level we can TORA, OAT, ATOW, etc.