For the same reason you would use "obliged" vs "thank you". In Portuguese, the prefered way of expressing thanks is by saying "obliged". But the verb "to thank" also exists: it's "agradecer". And "thanks" as a noun is "agradecimentos".
So annoying... in the other sentence with this verb, I said, "gives thanks," and it marked it wrong, saying one of the translations was "says thanks." So of course I reverse my answer on this one and still get it "wrong."
Maybe if someone is praying they might say "I'm thankful for the meal" rather than saying "thanks for the meal", which is more informal (if it's a prayer, or something like that, then it should be more formal).
It did not accept "I appreciate the meal" It said the answer should be "I thank you for the meal" but the sentence does not indicate the second person on it. In that case it should have read "Agradeço você pela refeição"