Google is in the business of serving ads. The vast majority of those spammy sites display google ads. So, there is little incentive for google to change things just yet. Joe public is not complaining, yet. It is only the digerati, and a whole bunch of other webmasters who think their sites should be ranking higher because they are just better, who seem to be up in arms over this.
The interesting thing is that half the spammy do provide content that is 'just good enough' for what most people are looking for. The quickest exit from one of these sites, is via a google ad, so google wins, the site wins, and the advertiser targetting a specific niche wins.
The only problem will be when the advertisisers stop getting the bang for their buck when their ads are displayed on these sites. Until then, Google has little incentive to change.