Why "arrives" is used with 'she' in a future tense.

No! In future tense, we can't use "arrives" with she or any other subject. We use this type of very only in simple present tense affirmative sentence when there is as a subject he,she,it,any single name or singular subject. Using this type of verb with these subjects is to make that verb in singular form because we all know, a singular subject takes a singular verb and a plural subject takes a plural verb.