Asking why this should be the case is not particularly useful. The fact is that it is true. If you offer an average adult cereal in the shape of little ronald mcdonald faces vs. generic flakes even with identical nutritional content they will, on average, select the more 'adult appropriate' option. Marketing is a thing for a reason.

The parent was alleging the opposite: that we are infantilizing ourselves by choosing child-oriented food shapes.

I think this is only a problem in a subset of cultures, not one of "adults" in general.