It doesn't have to be! It's just a matter of having the "Ideal" temperature. Cannabis grows best when the maximum temperature is about 80 degrees f. , that's just the way it is! Penguins may like it colder and cactus probably grows better when it's hotter - but cannibis likes 80 degrees f. as a max temp! If you vary the temperature by a few degrees (hotter or colder) it will affect the growthrate in a negative way.
Let me put it this way - you know those plants in your area that you said grow "huge" at 90 or 100 degrees f.? If the temperature never went above 80, or so, and maybe down to 70 at night - they would be "Friggin' Trees"!!! It's all about the difference between "So-so conditions" and "Ideal conditions".
Actually, cannibis can deal with higher temperatures quite well, as long as you boost the co2 level up a few notches. But then you get into a higher metabolism rate, more nutrients, more frequent watering, etc.
. When its too hot the plants have problems transpiring water from the leaves but the outdoors plant(s) have something we cant reproduce inside and thats the sun. The sun gives off vitamin D and other stuff thats very beneficial to both plants/animals/people, thats why those plants grow huge and what not.
