The Confusion Of Then And Than

In the last ten years I've noticed that many, many people write then instead of than (with a smaller amount occasionally using than instead of then).

This seems a bigger problem than simply hitting the wrong keys, though most people will claim that's all it is when called on it.

Could it be that a sizeable chunk of the population aren't aware that the words aren't interchangeable?

I see that there have been other questions on this, but I want to ignore discussion over whether they can be used interchangeably (they can't), and focus on the reason the misuse of the term seems so prevalent.

EDIT: To further give focus to my question, I know a few employers who are clear that the way people write their applications will immediately affect the applicants' chances of even being considered. Among other issues (normal spelling, grammar), they have identified that this mistake will cause them as prospective employers to make a judgement on the level of education (or even intelligence!) of the applicant (rightly or wrongly).

The confusion between than and then was much worse in the seventeenth century than it is today, it seems to me. Consider this example from page 170 of Ricard Baxter ("a Hater of false History"), Church History of the Government of Bishops and Their Councils Abbreviated (1681):

They taught the people, they bred up young Ministers, they kept out Heresies and Schismes, they guided the churches by the light of Sacred truth, and by the power of Reason and Love: And who than was the Bishop? who is the real Architect he that buildeth the house, or he that hath the title, and doth nothing unless it be hindering the builders?

And these contrary examples from page 461 of the same book:

There is a clearer word in the Gospel for the Ministry then the Magistracy; though enough for both. Our own call I shall speak of anon.

... Now a wound to the stomack or liver is more mortal to the body, then in the hand; and the loss of an eye or hand is worse then the loss of an ear.

... How can they more surely ruine Christs family, then by casting out the Stewards, that must rule, and give the children their meat in due season, even milk to the babes, and stronger meat to them of full age, [biblical cross references omitted]. What readyer way to ruine the Schools of Christ, then by casting out the teachers that he hath appointed under him? Or to ruine his Kingdome, then to reject his officers? Or to wrong the body, then to cut off the hand, and pull out the eyes, or to destroy the principal parts?

Here, in roughly half a page of text, Baxter uses then for than seven times. And yet elsewhere in the book he uses then for then and than for than most of the time. It is really quite mysterious.

Baxter is by no means unusual among writers in the era before Samuel Johnson in his tendency to drift between then and than as the mood strike him. Google Books is full of examples where similarly wobbly usage prevails. Underlying the phenomenon, I believe is the great similarity in sound between then and than in spoken English. Perhaps what we see in Baxter and his fellow writers of the 1600s and early 1700s is not confusion about how to spell the word that today is spelled then and how to spell the word that today is spelled than, but a consistent effort to spell the word the way it sounds in a particular instance—with the word signifying what we today mean by than sometimes pronounced than and sometimes then.

If nothing else, the example of Ricard Baxter will, I hope, persuade you that writers have walked an unsteady line between then and than for far more than the past ten years.

I suspect that it's a combination of factors. It's not all that unusual for a well-educated person with near-perfect English to, due to a simple "typo", use the wrong spelling when typing a homophone (especially online, where proofreading/editing is minimal). It's not at all unusual for someone whose native language is not English to get the words confused. And it's fairly easy for anyone to slip into the habit of using an idiom with the wrong version of a homophone, if they don't think about the source of the idiom.

I've not noticed this situation getting any worse over the years (though I haven't noticed it getting any better either). But certainly one may be exposed to it more with more unedited online content.