The English spelling system is amongst the silliest in the world.
I make a few spelling mistakes occasionally. I usually spot them and put them right. That's usually a result of typing too fast.
As for other people, some people have genuine problems with spelling - my brother, for example, has very bad spelling and always has. That is a form of dyslexia, which occurs more amongst English speakers than speakers of languages with a more phonetic orthography. I have met one or two 'dyslexics' whom I wouldn't categorise with my brother, who is, believe it or not, possibly more intelligent than me.
Some just never bothered to learn to spell properly when they were at school and somehow still get away with it later in life. Some people are just plain stupid.
It's not just spelling - punctuation seems to cause even more problems.
i blame poal's
I just added that for a bit of fun.
By the way, Irish Gaelic seems to have quite a silly spelling system, and don't get me started on French!