Because good materials cost more than "fancy" decorations.

When hotels buy doors, they optimize for things like cost, fireproofing, and weight (so guests and housecleaning can open them easily). Noise cancelling is a nice-to-have, but if you're buying and maintaining ~100 room doors (USA average per hotel), then you have to cut costs somewhere. On top of that, hotel hallway environments tend to be noisy in the first place. We're talking long, narrow hallways with hard walls, plus constant streams of holidaymakers who aren't exactly tiptoeing around.