Why does my newborn startle so easily?

Yup, like what was mentioned, your baby is exhibiting Moro Reflex. It typically happens when baby’s head changes position abruptly, when he/she hears a loud noise, or when baby experienced a sensation of falling (e.g., when being lowered into the bassinet with a lack of support). There’s no need to worry about this and it will decrease with time. You can try providing extra support when transferring him/her from one place to another to minimize the occurrences. If your baby frequently startles himself/herself awake, you can try swaddling to see if it helps. :)

Until he's 2 months old, he may respond to loud noises, sudden movement, or changes in temperature (such as when you remove his diaper) by throwing his hands out to the sides and his feet up in the air and opening his eyes wide. This normal response to a new or uncomfortable sensation is called the Moro reflex. The Moro reflex tells us that a baby's nervous system is healthy and functioning normally.

They're just getting used to their surroundings, their bodies and their feelings. You know how a young baby finds peekaboo very entertaining because they don't have a concept of time yet, it's the same. They don't really have a grasp on their surroundings yet and everything stimulates them,hence the constant bouts of "shock".