Kann sich jemand einen Grund dafür vorstellen, warum so viele lateinisch- und griechisch-stämmige Wörter im Englischen fälschlich in der Pluralform verwendet werden?
Because the words are English words of foreign origin, no longer actual Latin or Greek words. English speakers borrow readily and adapt the words to the rules of English, whether those rules are phonological or grammatical. Most Americans under the age of 50 have never learned Latin or Greek and neither know nor care about the origin of data, visa or alumni. For them, these are English words, and I can't fault them for assuming they follow the rules of English. Data doesn't look plural (no -s) and datum is practically never used, so it's not surprising people say and write "the data is."
The same thing happens in German, as others have mentioned. Thus Bonusse and Boni are both found as plural forms for Bonus.
So few words in English mark gender anymore (teacher, doctor, lawyer, soldier, driver, player, coach, governor, senator: all can be a man or a woman) that expecting people to maintain a distinction between an alumna/alumnae and alumnus/alumni will surely lead to disappointment.
Und was kann man dagegen machen?
Not much. Maybe take a deep breath. Where both forms are in use (a phenomenon, a criterion), I see no reason you shouldn't stick to the form you are comfortable with. Unless you are a teacher or editing someone's scholarly writing, however, I wouldn't correct mistakes like "a phenomena." As Martin--cal said, "most people are not going to appreciate being corrected, especially if you are a stranger, and even if you are a friend." I could get upset every time someone said "5 pages or less" or every time I entered a grocery store and read "10 items or less," but that would probably just give me high blood pressure.