I don't think that's entirely unreasonable. After all there are hardly any personal incentives for individuals to invest into infrastructure, education or healthcare of people who can't afford it and plenty of other areas even if that's what allowed them to accumulate a significant proportion of not the overwhelming majority of their wealth over the long term.
Given that most taxation done with the the advertised goal of helping the poor in Australia does happen with popular assent of individuals, I would theorize your position is false and that people do have some individual incentives to develop services offered to the poor -- for profit, humanitarian / charitable reasons, and a variety of other considerations.
I never said that wasn't the case but historically that "some" was never sufficient. On average people are rational and selfish to a larger extent than they are altruistic.
> taxes in that pursuit are illegitimate under a theory taxation happens by the assent of the people
Well the whole concept of an organized society falls apart if individuals can personally freely chose which laws to obey and which taxes to pay. You have to have a balance based on a reasonable consensus, otherwise you end up with totalitarianism or anarchy (and in that case the people who have the means and resources to do that will establish alternative power structures and will end up subjugating those who do not AND also outcompete those how have the means but are unwilling to do the same).
It can't simultaneously be true that most don't want to help the poor at their own cost while also the tax has been legitimized by the majority wanting to help the poor at their own cost. Only individuals or representatives elected by individuals can vote and if their incentive was to not help the poor then they'd vote not to and that would be that.
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.