> I agree, back when the UK was complaining about Google I was hoping that would push them into unifying tax policy.

It's not just tax policy. The countries that have less well educated workers or less wealthy consumers want to give companies some incentive to come there. If the tax rules were the same they would use lower rates. If the rates were also the same then would use the tax revenue to fund subsidies or projects that act as de facto tax breaks.

Your choices are essentially "countries compete with each other for investment capital" and "world government with completely uniform laws." And the second one is Very Bad: Monoculture, nowhere to run if it goes wrong, single target for corruption and capture, etc.