In many dictionaries, such as Merriam-Webster, the adverb "often" is defined as follows:
often (adverb): many times
However, this adverb can be used in some ways different from the other adverbs of frequency such as frequently, sometimes, and rarely. For example,
How often do you call your mother?
He travels more often than I do.
He calls home that often.
I know the meaning of the adverb in such examples; however, I wonder why the other adverbs of frequency cannot be used in such ways. Is there any specific grammatical point here?