Not everything is a transaction; and not all transactions are processed serially in non-overlappend time slices. Most systems will process multiple pieces of wo

Not everything is a transaction; and not all transactions are processed serially in non-overlappend time slices. Most systems will process multiple pieces of work concurrently (and can't just throw a new OS thread at each of them). The implication is that you can't just back out of a call stack like in a quickly hacked-up script. Stack frames aren't containers for whole transactions. Rather, pieces of work are tracked in explicitly allocated data structures, and worked on during more than 1 function call. The exception model is not compatible with this.