All that gitters is not gold?
We know that the sentence means something which seems valuable at first may turn out to be worthless
Why is it all instead of some??

Question

All that gitters is not gold?
We know that the sentence means something which seems valuable at first may turn out to be worthless 
Why is it all instead of some??

Answer

"All that glitters is not gold" means that not everything that glitters is gold.  Some things that glitter are, of course, gold, but not all things.

Answer

There really is no definite answer because this is a matter of personal style and flavor.  Saying 'some' instead of 'all' might mean the same thing, but it wouldn't sound as poetic in my opinion.