Because, even when it’s spoken by over 2 billions people over the globe, it’s still frequent to have something vaguely resembling arabic printed instead of actual arabic, Is that Arabic helps you to check you're not writing gibberish, so you're not ending up as a bad example on notarabic.com.

I highly recommand the 18 minutes « crash course », it's very funny and you’ll learn a lot if you don’t already know the arabic alphabet.