But "Why don't we ...?" is also used as a suggestion, rather than literally inquiring after a reason.
"How about eating pasta?" "What do you think about (eating) pasta?" "Why don't we eat pasta?" "Shall we eat pasta?" "Let's eat pasta!" are all more or less synonymous.
I wonder why the mı is after the verb here. After "makarna" (which was not accepted) I would expect it to mean "shall we eat pasta?" (As opposed the strawberries or hats) and i would expect that after the verb it would mean "shall we eat pasta?" (As opposed to using the pasta to make art projects, for example....) obviously I am wrong, değil mi? But I don't know why.