Why is there such a flagrant disregard of proper spelling and grammar on many internet forums?

I have observed in many online forums that a good number of people don’t use proper spelling and grammar. I am not so surprised when I see this of younger folks, but when an educated adult can’t spell and has a huge number of grammar mistakes, it makes no sense to me. Sure, we all occasionally spell a word wrong or miss-capitalize a word, but I don’t understand why it happens so often.

What do you think?