I think that we have treated them as mortal enemies for so long that no one stops to think about why, and how we got there (the US is not innocent in that, although it was a while back!).
Starting with the Obama administration there have been extensive attempts to “make nice” with Iran. This included unblocking frozen funds and even sending pallets of cash.
It's a measure of one's civility not to murder indiscriminately, as the houthis
are prone to do.
Iran's government supporting houthi thugs put them in the same category.