Why do official Lego sets sometimes use 1 large part when several would work just as well?
Some of the reasons (including some you've mentioned):


To make building easier / sturdier, so that the set is more suitable for its intended age range
The larger pieces are sometimes cheaper than the component pieces would be
The component parts may not be part of the current inventory of parts available to designers
To make the build interesting
To provide an interesting variety of parts


In the case of the set you mention (and this is pure speculation) it is possible that 1x1x5 bricks are more expensive to make or that they were unavailable in the right colour at the time the set was designed. At the same time, the designers might have felt that the 1x2x2 bricks reduced the part count to the extent that the set was viable for the intended market.