I don't understand

I'm going to San Diego for work tomorrow morning (I really don't want to go, but that is whole different story), so I just called for a taxi to pick me up at 6:45am. The dispatcher says "no, sorry maam, earliest taxi is 7:15am". Perplexed (since I've taken a taxi from this same company early in the past), I ask him why and that I have to call another company because I have to get to the airport at 7 in the morning. He says "you call 5am, I can dispatch". Hello -- what? So basically, and I clarified this, I can't call a taxi 8 hours before I need one because they are booked until 7:15am, however, I can call starting 1:45 minutes before I need the taxi to get one....it doesn't make any sense!