Writing to /tmp is great if your file is loaded entirely in memory. Not all programs do that for a variety of reasons. Like I said, it's the tons of edge cases that make "constantly save" problematic and far from a trivial feature.
/tmp often is a RAM based file system. Not the best destination for data intended to be permanently saved. (n)vi is primitive enough to insist in copying the whole file (usually not a problem with files meant to be edited interactively), but nothing stops a more sophisticated program to store only changes (transactions) and consolidate on request. Most software, certainly all interactively used, ought to be "crash-only" software [1].
