Why a theory of physics can't be falsified

That's because it is not a formal theory.
You don't have any proof that it is saying
what you think it is.
In practice - you "falsify" newtonian mechanics,
because you want it to be falsified. You're
simply asserting  that it was saying against
your experiments; then you do your usual
arm waving  and throw shit at those who
question that.
And, of course - Euclidean geometry can't be
falsified too, but for different reasons.
So, against it, you're doing the same.