why not ja będę kupić?
I can't answer why
, but at least the rule is simple: perfective verbs form future tense by "normal" conjugation (kupię), imperfective with the auxiliary verb "być" (będę + kupować).
It's like asking why (in English) you make past tense of some verbs without the auxiliary ones (Did you buy/see/go ? etc., but Could you? Were you? etc., you can't say "Did you can?" nor "Did you be?")