It is only asserted the purpose is "tax evasion." It might be harassment evasion.
On taxes, the government can summarily seize any assets and accounts they want to, with the burden on you to prove you should get it back. Money is not safe in banks or any other form the IRS can identify.
The level of paperwork required is also massive. For example, a new rule requires filling out a form each and everytime a business pays someone $600 or more. For some, that is a massive level of paperwork as it requires a seperate form for each expenditure each time. Any bank transaction of $10,000 or more is automatically reported to the DEA and a special form must be filled out stating the purpose of the expenditure.
The greater reason might not be to avoid taxes. It might because they do not want to be forced to be essentially employees of the IRS, to live for the IRS and tire of receiving dozens of threatening letters demanding more information and forms a day.
They also may have tired of lawyers and incessant threats and demands from them too. It may not only because of taxes. It may be because they want to live an easier, more secure and less stressful life.