Wireless is not always cheaper than fiber. The cost depends on geography and spectrum availability. Wireless is also more expensive per bit. Wireless also has a shorter lifespan than fiber. All these are things that have to be taken into consideration when determining which has the lower total cost of ownership over the lifespan of the asset.
And they waste money in the long run as a result. It's the same situation -- pay once to put utilities underground and then you don't have to pay forever fixing the fallout from every different kind of weather.
My neighborhood, in a part of Maryland where I can get to horse farms in five minutes, has fiber. It didn't even have public water/sewer until two years ago, but it had fiber. Tiny, non-to-code lots, arial power lines, and easy-going permitting authorities played a role in making fiber deployment feasible.
That assumption only holds if the cost of the inputs do not rise with time. In fiber builds up to 80% of the costs are labor. If/when labor costs rise more than the cost of money then you are worse off putting off an inevitable/intended investment.
Furthermore if putting off an investment causes duplicate costs (build aerial, later replace with buried) or causes you to forego OPEX savings (aerial vs. buried facilities maintenance costs), then you are actually worse off by paying later.
Thus the time value of money is not the end all and be all.
Most of the country doesn't bury utilities.[1] Point to point wireless is a lot cheaper than burying fiber, and also a lot cheaper than maintaining aerial fiber that can be torn down by storms/tree branches.
[1] My fiber line hangs free from a telephone pole and is secured in the middle with a bent nail. But hey, that's one of the reasons my neighborhood in nowhere Maryland has fiber, while most of Silicon Valley does not.