1. Fiduciary responsibility - Corporations have a financial objective to minimize their tax burden, in order to maximize shareholder value. Sure.
2. Legality. There are numerous loopholes that are technically legal. And it's remarkable how much gray area there is when it comes to tax law. This is why you have the big teams of lawyers to figure out how to best game the system and avoid taxation, while just barely staying on the legal side of the line.
3. Legal Consequences. And if they go to far and get caught breaking the law? No big deal. They can go to court and then settle. Sure, there will be a fine, but they knew that was a risk.
4. Jurisdictions and Multinationals. Countries can only make laws that take effect within their borders. But MNCs operate across multiple countries, and can effectively play jurisdictions off against one another.
5. Spirit of the Law. While lawyers may argue what they're doing is technically legal, it is certainly against the spirit of the law. The law did not intend for corporations to shift all the profits from where they are made to low tax haven jurisdictions.
Governments are trying to address these issues, as corporations will be doing their best to stay one step ahead of the law. This is why the OECD has started the BEPS movement to come up with a coordinated response. One of their initial goals is to create some transparency, so they can at least get some visibility into what's going on. That seems like a good first step.