Why is my baby so gassy?

Depends. Babies pass gas that they swallowed during feeding or fussing or because they make it. We all have germs in our digestive tract, and they live off what we don't absorb during digestion. Some kids pass more because they fuss more, or eat more air when they feed. Some don't digest some of what they eat, like lactose in milk, which can lead to more gas.Eliminating lactose feeds helps some.
Because its a baby.. Babies all have gas issues. Some more than others, but gassy in general. Think of it this way. Babies can't walk or talk right away and we dont expect that. So why do we expect them to be expert poopers, farters and burpers? They know they dont like gas, but they dont know what to do about it. Every day feels like being in a car and having to pee with the next exit in 20 minutes. Fussiness...
Gas is swallowed air. (mostly), so if the baby has a lot of gas, more effective feeding and more frequent burping may be the answer.