Why are the boiling points of aldehydes less than those of ketones?
It isn't that good a generalization: always look at the data first.

Here is a table of most of the aldehydes and ketones with 6 or fewer carbons (the labels are used in the chart later):



Now plot this on a chart:



Branches is the number of branches in the carbon chain.

Note that while for 3 and 4 carbons the ketones do have higher boiling points, it is not clear this is true for 5 carbon compounds and certainly not true for 6-carbon ones.

So, I'd argue the pattern of boiling points is complicated and there is no simple pattern that needs to be explained.