It is indeed a mistake that is almost only made by native English speakers, which is in itself an interesting phenomenon.

IMHO it's because you learn your first language spoken first, written after -- and most people learn further languages after they learned writing, so they can learn to differentiate between words that sound similar/same.

Perhaps it's because native English speakers more frequently communicate with English-speaking people in an informal setting (where such mistakes happen more often and are copied), whereas non-native speakers tend to read English mostly in edited publications etc. and (more sloppily) communicate with friends in their native language.

While it is true that yes, people in general communicate more sloppily in informal settings it doesn't mean they don't know the formal way of writing (at least for common terms)

German dialects are pretty far from the "official" language, but everybody knows that

You're assuming its vs it's is always due to lack of knowledge. Sometimes it's just a brainfart.

I noticed today that I had sent an email that used the wrong form of "to" ("too" instead). This isn't because I don't know English. It's just that what came out of my fingers was wrong. Mea culpa, it happens.

The way English uses the apostrophe is especially tedious and offers no value: it'd be no real shame if the distinction between "its" and "it's" disappeared over time, just like English lost gendered nouns and words like thy and thine.

Oh brainfarts and autocorrects do happen, but by the frequency I see this mistake happening I see it's not just a regular slip

You probably see a lot more English than other languages, so you'll see a proportionally higher number of mistakes too.

Another hypothesis could be that in the culture surrounding other languages (as Russian or German) grammar and spelling mistakes are much more frowned upon than in the English-speaking countries.

Anti-intellectualism in general is an old and interesting phenomenon in English-speaking countries.

Note that even people who fully know correct usage do type the wrong thing now and then. It's particularly easy to do when you're basically transcribing a sentence that you're verbalizing silently to type the wrong homonym. I very rarely to never mix up things like loose and lose but I will mistype things like its and it's from time to time even though I fully know the correct one.