For more-less exactly the same reason you accept general relativity over aristotelian motion - it is derived from first principles using maths, can be shown to match experience even if somewhat intuitive to people, and works pretty well in practice.
> can be shown to match experience even if somewhat [un]intuitive to people, and works pretty well in practice.
I think these are the two points that those skeptical of utilitarianism have trouble with: it's exactly that it doesn't seem to match experience that started this thread. Additionally, it doesn't actually seem to work well in practice: http://econlog.econlib.org/archives/2014/07/the_argument_fr_...
