Why do columns of staircase attract a lot of load? When staircase located at a corner of building where landing slab facing inner floor area.

If I understand well....

First of all we are talking now about vertical loads (such as dead and live loads not about lateral loads) when staircase columns are located at the center of the building they are more confined so the horizontal forces (not very significant forces in fact)  are transferred to the floor (diaphragm) so they don't affect columns significantly . in case the staircase at the corner and the landing facing inner floor then the columns near the landings aren't connect with the main structure and are less restricted so the horizontal portion of the stair loads will be transferred directly by them to the ground,  columns are subjected to some kind of moments.

seismic loads have more influence on such external columns for the same reason I have mentioned . these columns are separated from the main body of the structure they aren't connected rigidly to the diaphragm so they have their own response to the excitation.

this can be understood easily by viewing it as a truss the column holds the usual load by area method but in our case the stair act as the inclined member in the truss and holds farther loads so  the more the stiff the stair the more loads generated in the column