Modern presidential campaigns are absolutely saturated with money, and everywhere they spend has diminishing returns. You might be able to spend $50-100M in ads and get some value from it, but a second or third $100M? By that point you've reached every possible voter many, many times and each successive ad can be zero or even negative value.

Given the immense sums of money, little to no accountability on how it's spent, and a finite list of traditional spending targets I expect campaigns have dabbled in all sorts of unusual approaches.