The root cause of the whole problem is horses. Seriously.
A .30" calibre round is simply too powerful to fire in an automatic rifle from the shoulder, limiting it to crew-served weapons, and the amount of heating the round puts into the weapon is also pretty huge, meaning that you really also need a water-cooled or quick-change barrel to take advantage of it.
That means for an automatic rifle to work in real life, you need to use a smaller ("intermediate") round which has less muzzle energy and uses a lot less propellant to keep a lid on the heating problems. The problem is that these rounds are much less powerful than standard .30" rounds - and don't have enough energy to reliably kill a horse. That was a major consideration when the rounds were designed, since infantry were still expecting to deal with cavalry charges. It wasn't until WW2 that the advantages of a smaller round became apparent, at the same time as horses were finally replaced wholesale by machines.
So the fundamental problem is that the standard rounds available until the German 7.92mm Kurtz round came out are just too powerful for an automatic rifle to make sense.