There are actually many reasons why people might make grammar mistakes on a forum like this one, many are understandable:
1. It's an international forum, and not all are native English speakers.
2. Some people have dyslexia or another similar learning disability.
3. People are using mobile devices with an autospell that tries to anticipate what you are going to write. These systems actually drive me mad, because they are supposed to help you, but you end up making ten times more mistakes than you would do otherwise. I never use one, I always set it to manual.
4. People write to reflect the spoken word, which might not be standard or formal English.
5. Ignorance. People simply don't know how to spell something, or don't know the grammar rule.
6. People fail to check their writing and don't spot mistakes.
7. Typos that don't get spotted.
8. They know they have made a mistake, but can't be bothered to change it.
Numbers 6 and 7 happen to me quite a bit, sometimes 4, but not so often.