A long time ago, you can't really remember when, you went down there, into the basement, and saw something that scared you so much, you refuse to go back down there, no matter what. That is why that today the door is locked with so many locks you've lost count. You could only open the trapdoor if you absolutely needed to, as in, a nuclear apocalypse. Or...
An imminent meteor crash.
But that's entirely unlikely. Like, what kinda person has their entire house crushed by a meteor, it's like...
Oh right, meowrage...
Well, that couldn't possibly happen to your house, right?
Right?