It could be the default for non-interactive shells without causing this problem. Or we could have a more nuanced rule, where -e means "stop executing the current sequence of commands as soon as there is an error", where a "sequence of commands" is a single line in an interactive shell (so "false; whoami" would print nothing), or the entire file in a script.

The real answer is that this has not been the default in the time between shells being invented and this comment being posted, and so the squillions of lines of shell script out there in the wild keeping the world turning have not been written with this in mind. Making it the default now would break a lot of things.

With the benefit of hindsight, though, i would say that yes, this should have been the default in scripts. Oh well.