Can big agencies actually deliver??

Win the pitches/business at all costs, sign on the dotted line and within 3 months you go from best friend to outcast. So I ask you why do clients still fall for the bells and whistles and the fancy offices and the verbal diarrhoea? I mean smaller agencies will be more dedicated to your business, cheaper, quicker turnaround, better understanding of your brand. Skill sets definitely cannot be the issue, because most small agencies are run by ex big agency individuals. To the clients out there this is something you know and experience everyday, so why are you not doing something about it?