Because the weight of your breasts (unless they're really perky and stay up on their own) dictates that suitable 'scaffolding' is needed to keep them where nature intended.
Many women moan about backache and straps digging into their shoulders if straps are thin. Physically, it's not sensible to try and successfully lift a saloon car with a piece of string attached to a crane. I wear 36G or GG depending on style and more iften than not the back fastening is a 3 hook and straps slightly wider than on tge two hook version but at least there is no worry about back fat and I know clothes hang better on a good foundation. You might possibly find the odd style in your size with narrower straps in the Fantasie range. Debenhams and online. Best wishes. Oh...the Primark DDs are 2 hook narrow strap. T-shirt bras 3 for £10 but you get what you pay for. I prefer to pay more for well fitting lingerie which makes cheap clothes look better...IMO. ??
Margs, I just buy mens t-shirts for that exact reason, they're cheaper! Bras I find uncomfortable, so they're just for going out in, rather like shoes or coats.