English equivalent Polish saying "Dumb people are not sown, they're born"

There's no cure for stupidity.

Comes close to the connotation of the inevitability of nature the Polish one has.

The saying "there is one born every minute" is close to yours:

• There are many fools and dupes in the world.

Origin

• The earliest form of the phrase that I can find in print comes from The European magazine: and London review, published by the Philological Society of Great Britain, 1806:

• It was the observation of one of the tribe of Levi, [that is, a Jew] to whom some person had expressed his astonishment at his being able to sell his damaged and worthless commodities, "That there vash von fool born every minute".

• The "sucker born every minute" is widely said to have been conjured up by P. T. Barnum. Like many other sayings attributed to Barnum, they are just that - there's no evidence that he coined this phrase.

• Whilst probably not having been coined by Barnum "a sucker born every minute" is certainly an American phrase. The first record that I can find of it in print is in The New York Times, December, 1883:

• 'There's a sucker born every minute,' as the gamblers day.

(The Phrase Finder)

There is an English idiom that matches your "long explanation" almost exactly: "Dumb people are born, not made".

It is also used when talking about someone with an exceptional ability to do something (for example "Olympic athletes are born, not made") as well as to an exceptional lack of ability.