Yes, but if you demand that everyone pays proportionally to the benefit they get, then the threat of moving capital to another country has an outsize influence

Yes, but if you demand that everyone pays proportionally to the benefit they get, then the threat of moving capital to another country has an outsize influence on politics within the country. So the rich get even more say about how taxes are spent than they normally do, by literally voting with their wallets. And thus state’s spending is optimised to protect the interests of the rich with little regard for the poor.