Sidechains are not built "on top of" a layer one because they do not inherit the security of another blockchain L1. Instead sidechains are on "the side" because they rely on their own consensus for security (i.e sidechains are L1).
To use an analogy
Sidechains are a way to make a blockchain faster as much as taking the plane is a way make a F1 car's engine faster.
Sidechains are not built "on top of" a layer one because they do not inherit the security of another blockchain L1. Instead sidechains are on "the side" because they rely on their own consensus for security (i.e sidechains are L1).
To use an analogy Sidechains are a way to make a blockchain faster as much as taking the plane is a way make a F1 car's engine faster.