For many cloud VMs you spin up, it's the default. Having your servers behind a NAT not only requires a lot more infrastructure knowledge (you need to know you need it and manage access and routing), but also quite a bit more capital investment; i.e. you need to set up a full infrastructure compared to spinning up two+ VMs.

That's not to say it's a good thing, but I'm always surprised by the lack of deeper network knowledge by a lot of engineers (and that's not meant degrading - it's not something that you get for free when programming).

Lastly, you did probably start the project with a single VM - and at that point it's far harder to say when the point comes to move to a NAT, even more given that getting your second server is probably needed in a sudden spike and the switch is a lot of work with no immediate payoff.