Because nothing isn't something, and something is true. It's base Boolean logic where everything is either true or false. Null/nothing is false.

It's a weird way to think about conditionals, but it makes sense when you use them in real examples. In my case, I use them like this when I need to make sure that a variable has a value. So I can do something like

If(variable){do things with the variable}else{do stuff when the variable doesn't exist}

a  = null
if not a:
   …

if not a were null then an if that evaluates that would evaluate it as falsy… also if a would evaluate as falsy :/ that’s far weirder behaviour