I think
Pittsextra is making the point that it can (probably is) very quickly so complicated that the chance of a successful prosecution falls below the required 'proceed' threshold.
In a perfect world there would be unlimited resources to do this, in our imperfect world they have to choose what to do on the basis of 'where is our energy/money most usefully applied'.
I remember working with a TA colleague a few years ago, who's real job was HMRC, and he said exactly the same - they went for the easy ones because the moment it got complicated:
1. It absorbed vast amounts of time and money.
2. They nearly always lost.
Real world v. ideal world.