Simplistic/legalistic answer: because it's not required!
Originally Posted by 14CFR25, my emphasis added
§25.125 Landing.
(a) The horizontal distance necessary to land and to come to a complete stop (or to a speed of approximately 3 knots for water landings) from a point 50 feet above the landing surface must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane):
It's DEFINED at the standard temperature. So it's only presented at that one temperature.
That's not an answer to "how does the landing distance in practice vary with temp?" which is a physics question. But LFL (and ALD) are
defined quantities, and if the definition says not to consider temperature, then you don't.
(Just like VMC is not, despite the name, the "minimum control speed" - it's the minimum control speed for a precisely defined set of conditions. An aircraft, OEI, can definitely be "controlled" at a lower speed, by allowing the conditions to change. (It can also be uncontrollable at the same speed, if we make the conditions more adverse). But it's defined as a specific set of conditions, and everyone uses that same "level playing field" for consistency.)