Why a side chain token can never be counted on to be as reliably secure as the main chain

Complexity. On a side chain you have three separate codebases to protect from hackers and bugs: a host chain, a side chain, and a bridge. This can as much as triple the attack surface. Furthermore, any complexity you add to the main chain also affects the security of the bridge and thus side chain, but the same is not true vice versa.

So it is mathematically proveable, even assuming they are both decentralised and the bridge is decentralised, that a side chain token cannot be any more secure than the host chain, but the side chain token could and likely will be less secure.