The important rule for Swedish here is that the verb goes in second place in the sentence. So if you start a sentence with the subject, you must have the verb next, and then it will only be possible to have the adverb after the verb.
I wrote some more about word order in general here: https://www.duolingo.com/comment/8970470
I think it actually has to do with the verb phrases. Like you can't split "go home" or "sit here" because going home or sitting here is a single action phrase, so the sentences are either "I often go home" or "I go home often", but not "I go often home".
Whereas in "she is here", "is" the entire action (her state of being), and "here" is just a description so "is" and "here" are separate clauses and can be separated ie "she is rarely here". FWIW I think "she rarely is here" is also acceptable grammatically, just not in common usage so it sounds awkward.