On the contrary, this is an extremely non obvious, loaded assumption. It is deemed true in many cases, such as in front of a court, but is then well known to be a judicial fiction.
The worst problem with some people (particularly libertarians) is that they actually believe that this is the simple, unadulterated truth, which unfortunately isn't nearly as easy as this, and this will bring our civilisation to complete collapse under the oh-so-wonderful pursuit of our own individual self-interest, the same way its huge antlers doomed the gigaceros (for the individual male gigaceros, bigger antlers were always a net advantage against competitors in the "struggle for life", but they assured the species inability to adapt as a whole).
Annoying as it is, this is again best explained with the famous joke: for every complex problem, there is a solution that is clear, simple, and wrong.