1. Because it is a race to the bottom: if you're getting companies in there because of your 'near zero' corporate tax, don't be surprised if they move to another country with 'nearer zero' corporate tax, and lower payroll tax as well, and maybe poorer working conditions.
They're going to go where the talent is. If you're trying to set up a software shop, you're not going to have much success finding skilled employees in Zimbabwe or Kazakhstan; but there's tons of skilled employees in the US. Skilled employees rarely leave their country for crappier countries just for work, unless they're being offered a huge salary (like with the Americans who go to work in Dubai or Saudi Arabia). Having a lower corporate tax is good when you're (you=national government) trying to compete against other countries with similar standards and costs of living.
2. Because if a company isn't paying corporate tax, then it is much harder for it to be worth having them in the country (the cost of servicing their existence may exceed their return to society/government)
Did you forget that the company is hiring employees, who all pay income and other taxes themselves? The more high-paying jobs you can attract to your country, the more your workers (and imported workers) will pay in taxes. It doesn't cost anything to "service the existence" of a company, unless that company is creating a lot of pollution or causing some other negative side-effect. But in that case, you can deal with that problem specifically, such as by taxing pollution or pollution-generating industries. Software companies don't produce any significant pollution, and what they do, such as electricity consumption, can be dealt with with taxes on electricity generation (with different taxes for different types of generation: wind,solar = low tax, coal = high tax, etc.) to make electric power cost reflect its true cost to society.
In simple terms, when you tax something, it means you really want less of it. For most things, when you add a tax, you create a dis-incentive for people to consume that thing. Sales taxes discourage sales and commerce and consumption; property taxes discourage the purchase of property, even income taxes reduce the incentive to make more money unless you can do so with no more work. So if you tax companies, you're reducing the incentive to have and operate a business. Since economies depend on businesses operating and generating profit and employing workers, why on earth would you want to discourage people from doing that, by having taxes on it? There really shouldn't be any taxes on business, logically; instead, you should just tax income (since not many people want to make less money). All that profit that business makes eventually becomes someone's income, so there's no reason to tax it as a profit; that amounts to double-taxation. Of course, you do need to have some protections to make sure people don't just move the money offshore somewhere to avoid paying their income tax on iot, and it would help a lot if capital gains were taxed at the same rate (or maybe higher) than wage income.