The reality only changes what OTHER people see around them, it doesn't change what everyone (even the user) sees. Sorry, that was kind of confusing, but I'm sure you get my point.
another question: why didn't he make more materials from the infinity stones so that he didn't have to kill them... and why not use the reality stone and not only get the stones... but use the reality stone and save your one time snap for... I don't know. make a whole other freaking universe of what ever you want... or more snaps?
thanos: *uses reality stone to get other stones*
also thanos: touches the power stone first and nothing happens cause there not real
but i still dont get why he couldnt have like used the red stone to freak the avengers into letting there guard down then killing them one by one or all at once