Vienna was an international city; and Mozart and Shickhander were seeking international fame. The Magic Flute was a relatively low-brow 'singspiel' in the native language but its creators wanted foreigners to be able to see it and like it. (It was common for the wealthy and educated to speak many languages, not necessarily very well.) In music history you see critics criticize some operas for being difficult to understand, and remarking if the audience seemed lost.
International audiences nonwithstanding, it's just hard for many people to hear song lyrics, and a very common choice to make song lyrics simple, and hearing lyrics is critical for opera in a way it isn't if you're singing Goethe at a small salon concert.
The original point is it's silly to compare opera lyrics to spoken dialogue. Songs with belabored and repetitive lyrics can easily be interesting, spoken word with this property is banal.
