Most southern Yemenis don't want unification. North and South Yemen were separate countries for decades. When unification first happened there was optimism but South Yemen (which is about 20% of the total population of Yemen) was always hesitant about how little political power they'd have. North Yemen is 80% of the population and, today, almost entirely controlled by the Houthis.

I don't think native Yemeni are motivated enough to fight for unification. That's why all the fighters are mercenaries hired by the UAE or Yemen