So it should be free? The bank already has "money". It's already there so you can take it?
That's not how it works.
Do you not get a managed service where someone upgrades it, deals with outages etc? Are those people that work 24/7 free or is it another "already there"?
And there are things that come for free when you have instrastructure this big and expansive - one-time configuration and you either monetize it or pass down the savings and since every cloud service is in agreement that profits should be maximized you end up with cloud providers which have massive datacenters at very cheap cost due to economies of scale providing it at a value far exceeding normal hosting practices due to their ability to monopolize and spend vasts amount of money onboarding businesses with false promises which errodes the infrastructure for non-cloud solutions and makes cloud providers the only choice for any business as the talent and software ends up going into maintenance mode and/or turns towards higher profitability to keep themselves afloat.
nat is free to provide because the infrastructure to have NAT is already there and there is never anything maxing out a switch cluster(most switches sit at ~1% usage since they're overspeced $1,000,000 switches), so other than host CPU time managing interrupts (which is unlikely since all network cards offload this).
sure you could argue that regional NAT might should be priced, but these companies have so much fiber between their datacenters that all of nat usage is probably a rounding error.