The infinite circle
The government wants more money, so they up taxes, or create new ones
In doing so they create ways to avoid these taxes
Companies go "what? A 0.5% increase? That's an extra 10 million they want, bugger that" and pay to shuffle the accounts.
The government doesn't get the extra money they wanted, so they increase taxes
Overall the large companies and rich individuals can afford to pay tens of thousands to get an accountant to shuffle their funds and avoid 90% of the tax they SHOULD be paying. While the little guy (me and you) have to keep paying the ever increasing taxes because well... We can't afford no to (literally)
At the same time, the government can't drastically lower the tax rate to draw companies back to keeping their books accurate, because the public would have a hissy fit "Why cut their tax? They earn billions!" and there's no garuntee it'd make them keep more money here anyway.
At the same time, the tax laws are such a jumble it's become nigh on impossible to close up one loophole without either opening several others, or blocking off designed functionality which wasn't a loophole.
Really it's down to a case of, we have laws dating back to the 19th century, and although they started out simple they've been added to, edited, added to again, archived as a new document is penned to extend the original. What they need to do is stop amending the laws, and start re-writing them.
By and large, it would cost the government a fair sum to get all the lawyers etc around and to adjsut the system while the new laws (laws 2.0) are written up. But long term we'd probably be able to consolidate a few thousand different legal documents into a few hundered which go into further depth, apply to modern era problems rather than based on dated information and ideals. Furthermore because of the added simplicity to all these documents, they would (in theory) make it easier to enforce the law, monitor breakage of the law and just make things easier. Of course it'd probably take a decade or so of mess and faffing about to get these new laws in place, and then another decade or so for the country to adjust. But by the end of it all we'd probably be in a far far far better situation than we are in now.
But of course it'll never happen.