Yeah sure but we don't speak latin, English is a hodgepodge of a dozen other languages, both dead and alive, and the only value that determines whether something is correct in english or not is common usage. And octopi is used often enough to not really be incorrect. You might as well be complaining that "ain't" isn't a word despite it clearly being so ever since Dickens used it.
This is a specific case of trying to imitate the rules of Latin, incorrectly.
Another one that we misunderstand a lot is virus. It looks like the typical second declension thing ending in -us. It is actually a rare categorization of word, the 4th declension.
But it is a common occurrence in languages that when rules are complicated and confusing that they get generalized to simpler patterns from elsewhere in the language.
Romance languages have done that to Latin too! I gave an example of opera being the plural of opus. A lot of romance languages have reinterpreted neuter plurals as feminine singulars, because of that -a suffix.
