1. Maximize collections. This is what they're doing: spending their resources on cases with the best expected ROI. If you have $1T in net tax misses, chase the easiest 75% and expect a 90% collection rate.
2. Deter high absolute tax avoidance. This is what you're suggesting: spend their resources to make a statement, even if the ROI isn't there. If you have $1T in net tax misses, chase the hardest 25% and expect a 5% collection rate.
I don't think you're wrong, as there's value in deterrence (i.e., sue the hell out of rich tax evaders, and even if you lose, maybe the next one will be less likely to push it).