Girls get distracted by idiots who look over their shoulders, make sexist comments and metaphorically waggle their man-bits while the girls are trying to play. That is one reason.
Like everything, you get better at something by practising (regardless of whether you think computer games are worth it). The girl-gamers that are good at gaming, are playing in environments that are fun, supportive and non-sexist. That means away from you, OP. You think they don't exist. This phenomenon is called confirmation bias.
Edited to add: It does not matter in the least if the OP is male or female. Women are capable of carrying our own sexist commentary inside our heads. This is what the OP has. You think boys are good at computer games because they were born to it?
Why do you think the OP has decided to generalize her lack of skill at this particular activity to all women? She has internalized all the sexist messages she gets from people around her.
The third reason is that some girls have sussed out, while some men would never admit to being sexist, these men think that their man-bits will fall off if they lose to a woman. These men are prevalent in society in horrifying numbers, and they are our brothers, fathers, husbands and colleagues. We suck up to them, to some degree or other, because we have to. We mollify them. Their foolishness and our duplicity feed off each other. This state of affairs carries on to the next generation.