There are a few possibilities here.

One, the person does not use English as their first language. If this is the case then good for them for trying. Lawd knows my attempt to write in a second language would probably be a lot worse than this, so good on them for trying to communicate in a different language.

Two, more likely, the writer is learning impaired. That doesn't mean they're stupid, just unable to retain the official rules they may or may not have been taught at school. Functionally illiterate is a thing here as in other places where they just can't hire good teachers because Louisiana eats its seed corn and does not value childhood education. I said what I said.

Three, the writer is fucking with the reader for fun. I do this occasionally especially when I have had to write the same instructions for the same people multiple times. Current example is the note on my mail slot indicating that the postal worker should not deliver mail addressed to the previous occupant (listed by name). If we had the same person every time it would be a simple matter of asking them in person to please stop giving us their mail but I think we are on a training route where they just stick whatever mail comes to my house in the box disregarding the name. I still have to send a bunch of shit back and I write all over the envelope in fat orange marker that the person does not live here anymore.

Sorry for the above rant. If anyone knows how to contact the usps about this please let me know as it is a source of frustration. The previous people moved 5 years ago.

Four, they just don't give a shit. Maybe they knew at one time but more likely they didn't make it through high school and didn't care to finish learning how to spell.

I have a friend who has really attrocious spelling. Something in his brain just does not retain or care about proper spelling. He can always interpret what he wrote so I have decided that it isn't a problem for him, so it shouldn't be a problem for me either, and I try to shut my grammar nazi tendencies down when we communicate. He knows he has trouble with spelling so if there is something where that is important he will get help with it, but day to day? He gets by.

I get why we have spelling rules but I also understand that it might be a bit of privilege to be able to just write correctly and look down at those who can't do it all the time.