Because all our youth prospects have become middle aged, injury prone, indisciplined and not particularly good (Szczesny, Ramsey, Wilshere) with Bellerin the one prospect remaining (why isn't he on the list? Is he over 21?) and maybe Holding, if one were to be extremely generous (although I wouldn't put him in this bracket). Another legacy of the later Wenger years - youth replaced by middle-aged journeymen like Mertesacker and Welbeck.