Because so many people have used it as a single word for SOOO long that they feel it's a word via popular usage? Or, from what i found online...
"This spelling of 'a lot' is frequent in informal writing but not generally accepted by arbiters of English usage. Others view it as a legitimate contraction. Some occurrences of alot in print may be typographical errors.
1993, The Columbia Guide to Standard American English calls alot “substandard” and notes that it is “increasingly found in informal correspondence and student writing” and “has as yet received no sanction in print except on the op-ed and sports pages.”
1996, The American Heritage Book of English Usage states that 'alot is still considered an error in print' but notes that standard words have formed by fusion of the article with a noun, such as another and awhile, and suggests the possibility that alot may, like them, eventually enter standard usage.
2004, Jack Lynch Guide to Grammar and Style (entry dated 2004) flatly states this to be a two-word expression.
2004, The Cambridge Guide to English Usage also compares alot to awhile. It states alot to be 'still regarded as nonstandard' and notes 50 appearances in the British National Corpus, 'almost entirely from three sources: e-mail, TV autocue data, and TV newscripts.' It suggests that some usages of alot in typewritten use are to be considered merely typos of the standard a lot though its appearance in handwriting and typescript is “more significant, as the shadow of things to come.”
