Not only should it be, it is.
Oh, but I beg to differ.
Both "can't" and "gotta" are absolutely forbidden in academic writing and both are accepted in informal writing.
The mechanisms that governed creation of these words are also essentially the same. Both were created through lazy and hasty use of the language.
In case of "can not", the proper vowel of "not" was reduced to
schwa (as it is often the case in fast speech) and subsequently turned into a further reduced form of
schwa, which is typically represented by an apostrophe.
In case of "gotta" this mechanism reduced the proper vowel of "to" to the
schwa sound, which is represented by the final 'a'.
There is no possibility of having a seperate word with only
schwa vowel (it would indicate the word has no stress), so the reduced forms have to be connected with words that actually have long vowels.
And as both "can not" and "got to" are in common use, it was convenient to create representations of this hasty and careless speech through these two shortened words.
The only real difference between them is that "can't" is older and language users had more time to get used to it.
But hey, on the other hand... perhaps your personal attack was right. Perhaps four years of English philology really taught me nothing. Who knows?
