The reason the correlation between spending and winning is high is because people donate to politicians they think can win. If advertising spend could buy elections Mitt Romney or Michael Bloomberg would be president and politicians would usually be rich before they got elected, not as a consequence of getting elected.

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3042867

> the best estimate of the effects of campaign contact and advertising on Americans’ candidates choices in general elections is zero.

https://link.springer.com/article/10.1007/s11109-012-9193-1

> Because campaign spending correlates strongly with election results, observers of American politics frequently lament that money seems to buy votes. However, the apparent effect of spending on votes is severely inflated by omitted variable bias: The best candidates also happen to be the best fundraisers. Acting strategically, campaign donors direct their funds toward the “best” candidates, who would be more likely to win even in a moneyless world. These donor behaviors spuriously amplify the correlation between spending and votes. As evidence for this argument, I show that (non-strategic) self-financed spending has no statistical effect on election results, whereas (strategic) externally-financed spending does.