Probably because the law isn't either fit for purpose, or clear enough for someone to litigate.
As far as I'm concerned, if they (are obeying the law) and getting away with it, then articles like these only expose that the government are the incompetent ones.
No UK tax authorities use courts as a measure of last resort. They persuade companies to pay unpaid taxes. See, eg, Vodafone who came to a deal. These bills were billions of pounds.
http://www.telegraph.co.uk/finance/economics/9330102/HMRC-cl...
Although see also this:
http://www.theguardian.com/money/2011/dec/06/hmrc-tax-deal-v...
etc.
It's very expensive to sue these massive companies. It's probably a good thing that deals are made to reclaim some of the tax.