If the government wants a tax to be paid they need to make it simple and unconditional. If there are loopholes or ways to legally avoid it, they will be discovered and people will take advantage of them.
We just almost never talk about it in neutral terms: why was this policy implemented, what are the pros and cons, etc. Instead, it's just political talking points to get people to the voting booth.
It's true of course that there was an economic theory behind the policy. It's a subsidy; the government thinks it's important for the US to have more small businesses, and hopes that more people will set one up if the financial rewards for doing so are greater. Perhaps you could even find some business owner to explain why they would have stayed in their corporate job if not for the QSBS. But this subsidy could never have gotten majority support if it wasn't obfuscated behind the tax code.