> Though as far as I'm aware there's no inherent reason for this - apart from cost.
Cost and size. The bread-and-butter market for ARM is consumer electronic devices, where size really matters. Other chip packaging that's easier for hobbyists to work with would be a huge waste of space by comparison.
There are some ARM chips available in DIP packages, but I suspect that selection will always be relatively limited, since demand will always be relatively limited.
The ARM chips available in DIP packages are microcontrollers that you are more likely to see in a toy or gadget than a general purpose or multimedia computer. In embedded systems where they are used, you probably can't swap them out for another part anyway because of code changes needed, different pinouts, etc.
Cost and size. The bread-and-butter market for ARM is consumer electronic devices, where size really matters. Other chip packaging that's easier for hobbyists to work with would be a huge waste of space by comparison.
There are some ARM chips available in DIP packages, but I suspect that selection will always be relatively limited, since demand will always be relatively limited.