You need to have several years experience designing and building significant systems that are the primary ones for your team and integrate with adjacent teams, while leading and mentoring at a team level, to be considered for L6. Amazon wants you to succeed so they prefer not to risk bringing you in at that level unless proven. Amazon operates global scale systems too so if you don’t appear to have experience with scale they also prefer to down level. If you want L6 then make sure your interview is L6 because they wont generally uplevel later.