I've seen this quite a lot. It's when people can't be bothered to spend an extra two seconds or so to put an apostrophe with words such as don't and can't. Because it literally takes TWO button presses.

And now, a step-by-step tutorial on how to put an apostrophe when it is needed!
1. Type your word until the apostrophe is needed.
2. If on mobile, press the 123 button. If on computer, find the button containing the apostrophe and OH MY GOSH YOU DID IT AND IT ONLY TOOK TWO OR THREE SECONDS! But mobile users, we still have another step so read the step very carefully.
3. After you have pressed the 123 key, find the apostrophe and press it. AND YOU'RE DONE LIKE HOLY SHIT THAT WAS FUCKING HARD, WASN'T IT? ((//cough sarcasm //cough))

So people, stop the #missingapostrophe. Words like don't and can't need it to make them correct.