This, of course, is because Mike has edited his own past to conceal from himself the fact that when he was in grade school, he liked having Elly around to save him from the bogey-man. Did it matter to him that she made all that noise about how unfair it was that she was chained to the house while he got to roam at will? Not so much because he needed a hug and a kind word to keep the monsters at bay.
That being said, he isn't the only one who doesn't have a fear of a vacant Pattermanse. While they all to one degree or another insist that they don't need to have someone around to protect them from the creaking noises that they tune out when they're occupied with some sort of busywork, Foobs as a rule get horrified by every little sound and assume that monsters and prowlers and spider clown mailmen are lying in wait ready to eat their brains when they're alone and vulnerable. If there's someone close by, the horrible shapes in the shadows know to stay away so if that means that Mommy has to scream at you about how you hate her and don't want her to get anywhere in life, that's just something you'll have to obsess about when you talk to your therapist about how you feel as if you're why Mommy never seemed to get much out of life.