But we have to present ourselves worthily. The Catholic Church believes that in order to receive Communion, you must have been to Confession first (within at least 1 year) and be free of mortal sin. Otherwise, it’s sacrilege.
We refrain from allowing Protestants from receiving MAINLY out of Mercy for them, as we do not want them to accidently commit the sin of sacrilege.
The shared belief part is only a part of the reason why we don’t have open communion. But the main reason is to protect non-Catholics from sacrilege.
NOTE: Yes, there are MANY Catholics who have not been to confession receiving and they are committing sacrilege. That is a different (yet related) issue, but we shouldn’t be allowing non-Catholics to expose themselves to sacrilege just because uneducated Catholics do it all the time.
God Bless