I know google does try and combat this. For example any adsense account making more than x dollars per day does get investigated and if they're deemed to be too

I know google does try and combat this. For example any adsense account making more than x dollars per day does get investigated and if they're deemed to be too spammy their sites often get penalised in the indexes e.t.c. But adsense is only one advertiser, there are many more. And the problem is the measures taken to combat this are likely penalise new sites and new (legitimate) content. It's a difficult line to tread really.