Well, think of it this way:
Sometimes they have a pair and sometimes they don't. Like the verbs you mentioned: говорить/сказать are supposed to be a pair.
But! there is a bunch of words made from the same stem like
поговорить, проговорить, заговорить, разговорить, подговорить, отговорить, переговорить, недоговорить, приговорить...
(Enough for the 1st time I guess).
And each one of them has its own slightly different meaning than just "говорить" and its own pair:
поговаривать, проговаривать, заговаривать .... sure you get the construction.
And the same with сказать: рассказать, подсказать, пересказать ...... and the same about pairs:
рассказывать, подсказывать, пересказывать...
So technically, говорить is a root of the tree of the говорить-like words and сказать is like a root of the neighbour tree.

And I don't know why they (говорить + сказать = ) are referred as a pair. Maybe because they don't have their own true pair, like говорить doesn't have a perfective partner while сказать - is without "his" imperfective one, because the "сказывать" word is obsolete and is used veeeeeeeery rarely and by very old people only. So maybe people decided to get them married each other

So the thing to remember is that every word has its own meaning, its own history and its own usage and it can whether have an aspect pair, or not have one. And I don't know any reliable resourse to be able to make it clear. Except myself, maybe.
So, don't shy to ask and pick up words' usages -- that's what I have to say.


P.S.
Oh, yeah, говаривать WAS a word about 100 years ago:
"Говаривали в старину бывалышну о семи добрых молодцах ... "