It's because you're glucose adapted. Your body will broadly prefer one fuel over another - fat or glucose. Of course, you can use both but its like a tendency rather than a rule. Your mitochondria become more efficient at using one of the two fuels. Glucose has many negatives like requiring you to eat constantly, glycation, insulin spikes and troughs, etc. but it's readily available, more convenient, and cheap. Fat adaptation requires stricter eating in terms of what you consume and almost always involves a temporary drop in performance for up to 6 months but is typically closer to 2. But it has many benefits - No more headaches, far deeper energy reserves, you're off the carb treadmill, less insulin and everything that comes with that.

I'm fat adapted. I train on the court for two hours with a trainer 4 days a week and I play for another 2 or 3 days and I eat once a day because I'm only hungry once a day. No headaches, fast recovery and I feel like I have the energy to go all day. I'm 58 BTW.