Or people have a tendence to perceive a system where everyone is required to pay their "fair share" as more just and are more willing to (often begrudgingly) participate in it. If the system was fully voluntary the average contribution would be significantly lower that it is now. Also a significant proportion might feel they benefit from this system more than they pay into it (or are risk averse and prefer having the safety net even if they contribute more than they benefit)
> decided they individually are incentivized to help the poor develop infrastructure
Well I'm generally vaguely incentivized to help the poor and develop infrastructure. Would I be willing to voluntary give up 40% of my income to do that rather than a significantly lower proportion? No, of course not. When it comes to infrastructure I'd be willing to pay very little or nothing at all if I know my neighbour isn't contributing anything. I don't think the average person would behave particularly differently than me.
It's actually quite rare to find someone who says I will help the poor, but only if my neighbor also pays, otherwise they can get fucked. This is very bizarre thinking. The people in my basically ancap-dystopia neighborhood don't even behave in this sadistic way. It honestly sounds more like a manufactured "gotcha" to win an argument than a way someone would think.
In reality the fraction people that would vote to tax themselves, are more than willing to donate in the case they don't get the tax, your sadistic MAD view where you punish the poor just because some other guy didn't help too is relatively rare and not indicative of the mindset of most people who are voting to help the poor. I honestly think that most anyone who would vote for the tax, does not act that way when dropped in a neighborhood like mine, and you probably would not either.