I am of the notion that when you mention more than one adjective for a noun, you separate them with commas and finally an and before the last one.
That's partially correct, but not for all types of adjectives and not in all contexts.
If the adjectives are coordinate adjectives - ie adjectives that independently modify the noun and are of equal importance - then you separate them by commas. For example "it was a cold, gloomy, rainy evening".
As you can see, there is no need to use 'and' before the last adjective when phrased this way around, but if you write it the other way then yes, you need to - ie "the evening was cold, gloomy and rainy".
Interestingly, I was taught that you can identify coordinate adjectives using 'the "and" test'. Basically, if you can join two adjectives with 'and' instead of a comma without changing the meaning, then they can be comma-separated.
But cumulative adjectives should not be comma separated - for example, "a bright red dress". Cumulative adjectives build upon each other and are not of equal importance. The dress is "bright red", not "bright and red".
Also, when adjectives belong to different categories (eg adjectives of size, shape, colour etc) then they are not comma separated - for example "a big red ball".
You asked why adjectives are comma separated - I don't think there is an authoritative answer to that, but when you understand that coordinate adjectives are of equal importance and independent it makes sense that they are considered separately rather than cumulatively. For example, if you said "a white lace dress" it would mean the dress was made of lace that was white, whereas "a white, lace dress" means a dress that is both white and made of lace.