Until 18-24 months of life, babies don't understand object permanence. Without it, out of sight is literally out of mind. When hidden by her hands, a baby believes that its mother no longer exists. *Poof*, gone. This makes peekaboo a very dark game to play with babies.
While imitating your death (peekaboo), it may be depressing that your child would be so relaxed about your death. However, there is a silver lining; Batman's parents died infront of his own eyes, so every time you play peekaboo you increase the chance of your child turning into Batman.
Developmental psychology is one of the most enticing reasons to have a child. People tend to not like you doing experiments on their own kids, so one day it may be necessary to get my own. It is also more convenient to have your test subject living with you. My friends with kids would likely find it inconvenient for me to try and schedule their kids for regular tests of their cognitive development.