Because they need to keep some gaps in the tax law to allow their friends to pay less taxes than normal people.

The US famously had a huge amount of tax law exemption just because of corruption. Nowadays you need an international setup to achieve the same, because the same old tricks have been used by many and there was enough political pressure to change it.

Some countries simply just didn't go through enough scandals of finding out how all well networked people pay almost zero taxes and therefore still have some relatively simple setups to pay little taxes.

A separate matter is countries trying to desperately attracting businesses and creating tax benefits only for wealthy expats, but not for their own people.