Question: Why is there no pre-stall buffet in the landing configuration stall?
Probably because the stickshaker has been set to operate before natural stall warning occurs.
But it is also possible that the stall in landing configuration is not preceded by buffet. The low altitude pre-stall buffet occurs when the horizontal tail enters the turbulent wake from regions of separated flow on the wing. On some airplanes in some configurations that does not occur.
High altitude buffet is an entirely different mechanism. It results from a cyclic variation of lift when a shockwave causes flow separation which then changes the position of the shockwave. It can occur at any angle of attack. The angle of attack at which it starts decreases with increasing Mach number.
With increasing altitude (i.e. increasing Mach number for the low-altitude stall-IAS) the low altitude pre-stall buffet gradually changes into Mach-related high altitude buffet. That transition starts when the critical Mach number is exceeded, i.e. when the local airflow velocity exceeds the speed of sound. For the model shown in the graph above at an AoA of 21° that is at a free-stream Mach number of around 0.4.