Why don't sales people update the CRM and what can be done about it?

I spoke with a potential client about her company's lead management process.  When I asked about her CRM and how frequently sales team updates their database.  She replied flatly, “our sales team updates our CRM with varying degrees of commitment.”

I was reminded of an experience last fall, when I spoke at MarketingSherpa’s Lead Generation Summit, I watched presentation in which an audience member asked a great question. 

She asked, “I'd like to do lead nurturing/lead generation on behalf of my sales people but they don’t update our CRM - I don't have the information I need - so I really can’t do anything to help them.  What do you suggest?”

I think it was Anne Holland, who then asked the entire audience, “How many of you [marketers] have a problem with sales not updating your CRM?” Nearly everyone (170+ people) raised their hands.

Why do you suppose (despite our best efforts) many sales people seemingly avoid updating or skip updating contacts and notes in the database? 

I wrote a few thoughts on the matter a while back, “Where Lead Generation meets the Bermuda Triangle."