Scientific evidence here is considered uncountable. It is said it is because it cannot be "sliced up." Or that if you slice (cut in two) "scientific evidence" you still have a whole, not pieces. And yet this conception can change from usage to usage, and also over time.
Evidence has not always been uncountable, and some people still use it as a countable noun. And there is no Ruling Board of English that can legitimately insist they do not. In other words, if someone wants to use evidences as a countable noun, they are free to so. In this case, they are conceiving "evidence" as something that can be sliced in half and you get "countable" pieces of it.
Conceptions and uses wax and wane over the years. Proof can be countable or uncountable. As well, judgement can be countable or uncountable.
Gold is generally assumed to be uncountable, but it can be used as a countable noun. In a game, for instance, someone can ask How many golds do you have? Or How many coppers do you have? Someone may argue that this is just shorthand for units of gold, but I would not agree. It is actually using gold and copper as count nouns. This is solely based on the speaker's conception of the noun.
Coffee is often said to be uncountable, yet we can say How many coffees have you had today? We can also say The coffee of such-and-such a place is a coffee like none other. In one sense, we conceive coffee as uncountable, in another sense we conceive of it as countable.
In one sense, it is all in the mind. And it depends on whether we can slice the concept into countable pieces or not. And this differs over time and usage--and among languages. Language and language usage is always changing. Xerox is the name of a company, but it is also a countable noun. In fact, that dictionary entry says it is both countable and uncountable.
Here is another example: money. I just wrote that it is uncountable. But actually, it can be conceived as either countable or uncountable.
Last (for now), these same nouns can differ in other, related languages.
Information is classified as uncountable in English (I would not say: an information); it is countable in Italian (and the equivalent to an information [un'informazione]) is natural). When I encounter that, I experience something similar to the "it just doesn’t make sense" that you experience. I just have to put it into my mind that in one language it is not countable and in another language it is not.
Someday perhaps software will be countable in English; for many speakers, it already is.