Countable and uncountable are not absolute attributes - many nouns can be both countable and uncountable depending on the context. If the vast majority of the uncountable nouns are adjectivally specified, this makes the noun countable.
"Wine (uncountable) is a wonderful drink, but French wines (countable as it has been specified) are perhaps the best."
An uncountable noun represents (all of) a homogeneous group/set/class.
There are very few purely uncountable nouns (advice, goods, jewellery, and guidance are the only examples I can think of) but all countable nouns can be used in an uncountable manner.
Normally, we would think of "knife" as being only countable, but in "The pirates cut the bags open by knife." "knife" is uncountable. We know it is uncountable because all singular, countable nouns must be qualified by a determiner (a/an, the, his, this, that, any, etc.) and "by knife" has no determiner.