The English language has so many words that if you included them all, then common typographical errors would often match (by coincidence) a valid English word and therefore not be detected by the spell checker. Which would go against the whole point of a spell checker: To catch spelling errors. [via Raymond Chen]

Update 040409:
Today, I was killing some time and picked up “Writing Solid Code” by Steve Maguire in 1993. On Pg 3 the same point was made.