why it's PATH not set properly?

example% su bin

To become user bin while retaining your previously exported environment, execute:

example% su - bin

To become user bin but change the environment to what would be expected if bin had originally logged in, execute:

SYNOPSIS
     su [ - ] [ username [ arg ... ] ]

DESCRIPTION

        If the first argument to su is a ' - '  (dash),  the
environment  is passed along unchanged, as if the user actu-
ally logged  in  as  the  specified  user.   Otherwise,  the
environment  is  passed  along, with the exception of $PATH,
which is controlled by PATH and SUPATH in etc/default/su.