Two main reasons, and they’re both your fault.
The first is that you’re probably buying the wrong size — almost definitely too big.
The second is that you need to get clothes — suits, shirts, pants, basically everything — tailored. Almost nothing ever fits properly off the rack. Magazines have all kinds of tricks to make a standard-issue sample fit any model, including tucking, taping, folding, and pinning fabric to get the right fit.
Yes, that’s a trade secret, but it’s also nothing a good tailor can’t handle.