Or, some people hate having a pack on their back or around their waist, making them sweaty and uncomfortable. When I started mtb back in ’92, Camelbaks weren’t a thing; bottle cage and saddle bag was de rigeur. They only really came in after I stopped riding – when I got back into biking about 10 years ago, I got a hydration pack as everyone else used them and I thought that’s just what you did, but I never liked riding with one. About 6 years ago I randomly decided to stick a cage and bottle on for a short local ride, and that was that. Only wear a pack now for 4hr+ rides, and even then reluctantly.
In the same way that some people hate wearing a watch whilst others barely notice it, some of us just hate wearing a pack. Hence all the A answers!