It is not obligatory but highly recommended and it is not part of the pillars of Islam!

I need to know something. You know the caliphate in Islam is obligatory? But there is the Hadith about it only returning when Allah wills. So what are we to do in terms of bayah? I mean there are Muslim countries like Saudi but I know it is Haram to have multiple rulers. Please explain. I know the so called caliphate of daish is wrong