That's because to see is a non-continuous verb: https://www.englishclub.com/grammar/verbs-m_vmwct_1.htm
The Corpus of Contemporary American English tells a different story. You will find hundreds of usage examples for "are you seeing"? Only a very small minority have anything to do with romantic relationships or appointments.
The entire sentence "Are you seeing what I'm seeing?" is well attested in Google results. It fits well here: "Are you seeing what I'm seeing? Is that really 16 elephants?" "see" with continuous is particularly applicable when one isn't entirely sure if one can trust one's eyes or not.
Other parts of Duolingo seem to have no objection ;) https://www.duolingo.com/comment/226937/What-are-you-seeing No English-speaker in five years has objected to the English sentence as somehow unnatural.
People cite that example, but, in fact, that campaign dates only to 2003, a year that doesn't even appear as a positive inflection point in the usage of the various forms of "to be loving it" (cf. Google Ngrams). In fact, that's approximately the year "is loving it" starts tailing off after a vertiginous rise starting around 1985.
"You are living the life of a Tommy, and in your heart of hearts you are loving it" appeared in 1915.
"... and as we use it more, it is because we are loving it more, and the more we love it the more we get out of it..." in 1908.
"And to-day I am loving it with such intensity that..." in 1910.
"I could love it no more than I am loving it now..." in 1902 (granted, it's in a poem).
"Zien" in this sentence is the verb and "jullie" is the subject, as in "Jullie zien de olifanten". Or as a question "Zien jullie de olifanten?" I can't quite figure out how it could refer to "olifanten" unless you're thinking of "Zien de olifanten jullie?", i.e. "Do the elephants see you" in which case "de olifanten would be the subject. Does that make sense?