Gilgul

Question

The gemara in baba metsia (i think daf 107 amud 1)says that just like a person comes into the world without aveiros he should leave the world withuot aveiros. The rashash writes that this is a ktsas rayah that there's no such a thing as a gilgul. Could you please explain why it would be considered apikorsis to say like the rashash?Thank you.