A few reasons, 2 really noticeable ones. The first and most important is that in order to make that claim you need to prove it under scientific conditions and testing, which is expensive and time consuming. Product manufacture don't want to waste money on that if it's not their intended goal, especially considering most headphone companies put out new products regularly, and each one would need to undergo that testing.

Another important thing is that active noise canceling works not by blocking sound but by creating sound waves that destructive interfere with the outside sound waves. This means they (in theory) should be emitting sound waves of the same amplitude and frequency just 180° out of phase of the incoming sound to be blocked. While this would technically make the sound dampened, it is a much greater risk to the user because suddenly you have 2 loud sound waves being directed at your ear, and if the active noise dampening fails for some reason, suddenly they're not working to protect your hearing at all. This is why passive hearing protection is much safer than active. Passive is usually when a material just blocks the sound from reaching your ear at all.