Because kids have no respect for boundaries. My son is 9 and he'll just randomly some nights come climb in my bed...I'll wake up and there he is...I'm like dude, wtf are you doing in here, and he's like your bed is more comfortable than mine and I fell asleep playing (insert game here). I may have to get him his own PS4 because even though he has a Switch, and DS he still plays my shit.
It's really me and his mother's fault though...He grew up sleeping with us because by the time he was born we hated each other and he was our buffer......My daughter has ALWAYS had her own room and bed....I don't recall her sleeping with us even once....I mean I'm sure she did but I can't think of a single instance right now.