Governments should earn their taxes the same as a person should earn their income. Governments "earn" their taxes by making the benefits of operating in a nation worth the tax. Price fixing and collusion is illegal for companies to practice. Being a government doesn't remove the deleterious effects of price fixing and collusion. An international base corporate tax rate makes nations not care if the tax rate is even worth it to businesses. If the tax rate isn't worth it then a business will close or just never start.

Lets say that a corporation choosing a home nation is akin to you shopping for a good refrigerator. The refrigerator manufacturers don't like that one manufacturer is selling their equal quality fridges at 50% less than everyone else. There are two choices for manufacturers. Collude and fix the price of refrigerators or figure out what the 50% cut rate company is doing and try to compete. If the manufacturers choose collusion they can continue with business as usual without making improvements to cut cost or improve quality. But, they will cut cost, possibly sacrificing quality, as that is now the only way to increase revenue. The incentive to improve is removed and the risk of a refrigerator cabal outsider pricing at 50% increases. If the competitors instead choose to work on reducing cost or improving quality you as the consumer get cheaper higher quality fridges. Under the price fixing regime, when an outsider starts selling a fridge at 50% the fixed price the cabal either has to destroy them, bring them in the fold, or remove the price fixing.

With price fixing an outsider will always arise. Competition is the only solution that doesn't destroy itself.