Probably because of weird rules like "children's room" and "four cows' pasture", then the contrary notion that "it's" is incorrect for possessive form because "it is" already used for a contraction of "it is". English sucks.
I'm not a native English speaker and the fact that this is such a common error among native speakers baffles me because the rules that govern these things are very simple. Conventions about when to omit the 's' after the apostrophe differ from one set of guidelines to another (see e.g. https://www.dailywritingtips.com/possessive-of-proper-names-... ), but that's the only non-uniform thing about it.
There's nothing weird about "it's" being incorrect. Pronouns have their own possessive forms. You say her keyboard is white, not she's keyboard is white, and his shirt is yellow, not he's shirt is yellow -- why would you say it's collar is white?
You guys have no idea how easy English is, you should be happy about it and love it and cherish it forever! My native language has a phonetic alphabet, so it's easier to figure out how to pronounce words (i.e. no "lead men in battle" vs "the pipes are made of lead" bullshit). But forming possessives for plurals is waaaay more complicated than "add 's, except if the noun already has an s at the end".
Apostrophes are confusing and overloaded for several purposes.
His report card received four A’s and two B’s.
So, for plurals. Sometimes. Possessives, sometimes, and sometimes omitting the s after an s, other times not. And contractions, where ' is a placeholder for 9 or 10 different things.
""add 's, except if the noun already has an s at the end"
Texas's laws are conservative. (You'll find credible style guides with conflicting advice)
Right, forgot about A's and B's -- we don't use that grading system over here (although, FWIW, I've seen guidebooks that recommend As, Bs and 1s and 2s, because ABCs is also fine and the 1800s is a thing that has to be distinguished from the 1800s'). There's a lot of regional variation, too, which is inevitable considering that English is used from Brisbane to London and from Alaska to California.
But, seriously, every non-constructed language on Earth has exceptions that you have to memorize along with the rules. English doesn't have that many, and it doesn't have too many rules, either. I'm fluent in two other languages (French and my own), and I'm somewhat familiar with a few more languages of Latin, Slavic and Celtic descent. English is a breeze compared to any of them.
Perhaps it's because there's no phonetic difference that native speakers would not have learned the spelling or the rules at the same time, and thus make the error more commonly, due to not having felt a formalized understanding of the rules was necessary.
