That was the case in the past bécause it was believed that in right-handers it is the left side of the brain that is responsible for particular functions, whereas in left-handers the right hemisphere is the one responsible. This is called lateralization
Later on and with advances in neuroscience , it became evident that functions like language are still localised on the left side mostly even in left handed sample populations.
Now, the more acceptable sampling, is including them in the studies. Unless the studies truly need to exclude the variables and there are difficulties to find enough participants.
Last couple years I just go out , only a few wee little around now anyway . Well two guys showed up at the door with real beards . Yup I’m done ! Lol @OkieGemini