One of the very first things they teach you in college writing courses is that you should never use a contraction in formal writing (fiction being an exception). Part of the reason is because contractions are not formal, but part of it is also that if you use contractions a lot, it is easy to start confusing things like "you're" and "your".
I think, honestly, that many people have no idea what proper usage is, and for that I blame our educational system. I know that a lot of public school systems have changed their English programs since the good old days when I was in school. Now, instead of teaching a solid grammar and usage foundation, they try to teach things like how to write a proper business letter or resume. I think those things are important, but if they forget to teach kids grammar, of what use is knowing where the date goes in a block-style business letter?
You mention laziness, and I think that, too, is a component, but not in the sense you mention. I don't think people are too lazy to type things correctly, I think they are too lazy to pay attention to grammar rules and proofreading. And in all fairness, I don't think they are entirely to blame. Schools don't give a solid foundation, and society doesn't put enough emphasis on the importance of acquiring and keeping those skills.
Personally, I think the spell-checker program is the bane of modern civilization. People run the spell checker and figure they are good to go, not realizing that it only catches words which are misspelled, not instances of using the entirely incorrect word. I think the prevalence of technology-driven communication is also part of the problem. People who send and receive text messages on their phones frequently type "u r" instead of "you are" or "you're". People in chat rooms are typing so quickly that no one bothers to pay attention to the little details like grammar. Even keyboarding skills play a part. Once you reach a certain proficiency in typing, your fingers are flying very fast, and reaching that speed requires your fingers to memorize certain words and word combinations. If you aren't careful when you are learning to type, you end up repeating the same mistakes over and over, and homophones are a prime place for mistakes.
If it makes you feel any better, I am your companion in grammar whoredom :) Some of the other common mistakes that drive me batty are the frequent confusion of "its" and "it's" and the random appearance of apostrophes when there is no possession going on. The worst part is that it all seems to be getting more and more common, and people are minding and caring less and less. Several years ago, after finding four or five mistakes in the first two paragraphs of a lead story in our local newspaper, I wrote a letter to the editor about it. They didn't print my letter in the paper, but the editor was kind enough to call me on the telephone about it. The main point of his conversation was that in rushing to put out a daily paper, mistakes get made and things get overlooked. The main point of my response was that I understand about deadlines and such, but isn't that the type of stuff that editors and proofers are supposed to look for? I think the conversation was unsatisfactory on both ends, because I haven't written to him since then, and he hasn't ever called me back for a relaxing discussion on grammar.
On a final note, I should tell you that my sister-in-law is a high school teacher. She told me recently that the school district for which she works administered a pretty basic English test a few years ago. It was an English test that was given to seniors in her school district back in the 1960's, and which they were required to pass in order to graduate. She said there was quite a fuss in the school district because more than 50% of the teachers who took the test failed it.