Germany

Because hot dog buns taste like shit. Sugary like cake and a consistency of marshmallows. Germans like real bread.

Because they are fucking disgusting

Wurst is traditionally not really a street food. Bratwurst is to be eaten on a plate with knife and fork acompanied by potatoes and veggies of some description. Preserved Wurst like smoked Mettwurst is dry and can be eaten without holding it in any kind of bun. Boiled wurst like Bockwurst or Wiener Würstchen are cooked in a soup or stew.

So traditionally there was no need to have a fitting bun for the Wurst. Bratwurst in a bun is a relatively new thing (post war I'd guess) and germans just made it work with the buns that were available anyway. People got used to that, so nobody saw a need for any different kind of bun.

That's my personal take, so take it with a grain of salt.

The solution are Nürnberger Bratwürste. They are just as long as a Kaiserbrötchen and you get 3 at a time which fits perfect in the other dimension too.

As a German I think of American hot dogs as disgusting sausage wrapped in disgusting bread.

If the buns were longer, you couldn't eat the delicious pure sausage first. Best technique is to extract the sausage bit by bit by drawing it with your teeth out of the bun. As dessert you'll get a sausage-and-mustard flavoured bun.

That's not possible with hot dogs, as neither the sausage nor the bun taste on their own. I'd even doubt they taste at all...

Do you mean the shape of the bun or the kind of bread? Kind of bread: because American bread sucks. Especially hot dog or burger buns.

The shape is a more interesting question, I feel.

It seems to me that you are talking mainly about southern Germany, because they call the buns 'Semmel'. In the north it’s more commonly called 'Brötchen', which are more elongated than the round Semmeln. Other regional names and variants exist, too. A Brötchen isn’t totally Form fitting like a hot dog bun is, either. My guess is that the bun exists more as a vehicle for the sausage as opposed to a real part of dish, like in a hot dog. Some people just eat a Bratwurst mit Senf with just a small piece of toast to hold it, some of them even discard the bread. Its all about the Wurst, baby. So why are the buns tasty themselves? Might be traditional German quality standards. Back when they still existed.

Just my observation bit most Germans eat with knife and fork. Also, wouldn't work with Currywurst (think of a cut up sausage in enough curry sauce to drown a good size...we'll...it's too much sauce. Cheeseburgers...I've only seen them eat with knife and fork. But that's just my observations (limited as they are)