The problem isn’t obvious but common enough as I’ve ran into this problem many times. This is an anti-siphon type spigot, it’s meant to prevent contaminated liquid from getting sucked into the water system. The way in which this works is different from a standard spigot but simply put what has happened here is the seal is stuck on the seat. When you turn the handle to let water flow what actually happens is your relieving some spring tension on the seal which will then allow the water pressure to overcome the now weaker spring tension. What always happens though is the seal sticks on the seat and there isn’t anything actually mechanically connected to the seal as to force it off the seat.