For starters there’s so many performance enhancing substances, and entire underground industries for developing and producing them. All of these drugs would be banned by the blanket prohibition on unapproved substances, but in practice many of them don’t have any screening procedures, so won’t turn up in any tests.
There’s also the fact that many performance enhancing substances are biologically identical to substances your body is naturally producing (like bio-identical testosterone for example). The only way to test for these is to test the ratios of certain metabolites (with a margin for error to avoid false positives). In practice this basically means any athlete can do steroid cycles, as long as they are only boosting their test levels up to that of a genetically elite outlier.
You also have the therapeutic use exemption, which allows for legitimate therapeutic use of banned substances. Testosterone replacement therapy is a legitimate therapeutic use of steroids, and some sports are known for abusing TUEs for this usecase.
Testing for PEDs is way less precise than a lot of people think it is, and any motivated athlete can get away with PED use to some extent, no matter how much testing you subject them to. There’s good reason to be suspicious of a lot of athletes. One massively suspect category of athlete is all of the ones in/near their 40s who somehow manage to maintain physiques as good as/better than they had in their early 20s.