One reason is that the deals' valuations are so bonkers that banks' metrics for evaluating and negotiating deals are irrelevant.
Another reason is that the bankers are leaving banks for tech and the tech companies have their own in-house banking advisers.
Examples of deals without banks' involvement are:
Apple's $3 billion purchase of Beats,
Facebook's $2.3 billion purchase of Oculus VR this year, 2012 $700,000 purchase of Instagram, and $19 billion purchase of WhatsApp.
Oracle's $5.3 billion purchase of Micros Systems, and it's $2.1 billion purchase of Taleo
Microsoft's $8.5 billion takeover of Skype in 2011 and $1.2 billion 2012 purchase of Yammer,
HP's 2010 $1.7 billion purchase of ArcSight,
Yahoo's $1.1 billion 2013 purchase of Tumblr,
Google's 2013 $1 billion take-over of Waze, $228 million purchase of Slide, and $3.2 billion 2014 purchase of Nest.
Cisco's $590 million purchase of the Flip video camera. Cisco has bought so many companies - 170 - that it finds it cheaper to have its investment banking expertise in-house.
Mind you, when these deals are done, according to the Silicon Valley TV series, over mobile phones with valuations plucked out of thin air, why should anyone need a bank?