i believe there's a min assumed temp for the 737 you can find it in the QRH performance Chapter.
in my opinion ,the reason is there'll be no considerable gain when using an assumed temp lower than the min (ex min assumed temp at sea level is 30 deg C)
QUOTE (737.org.uk)
The normal range of assumed temperatures is from 30 to 55C. However the QRH shows that temperatures from 16 to 75C may be used. The maximum temperature is set by the maximum amount of thrust reduction allowable i.e. 25%; and the minimum is where the engine becomes "flat rated" and no further performance gain can be achieved.
UNQUOTE