Nitro,
To cut what would be a very long description short, the main reason that SOME break action single shot rifles can be chambered for hot cartridges is IMHO.....

The weight saved by using only one barrel instead of two allows for a stronger action design.

Heavy barrel lugs can engage with mating surfaces located either on the sides or within the floor of the action.

The slim profile of the one barrel allows the action to be heavily reinforced on either side of the barrel.
This can reduce flexing at the action bar.

Of course this type of strengthening could be done to a side by side double but would add much ungainly width and weight and turn the rifle into a club.