I mean, they do have the infinite money glitch, a.k.a. being owned by Microsoft. If Microsoft's investors think Fallout 5 will make its money back, it's more lucrative to get started on it sooner rather than later.

And it does also need to be said that they can't keep remastering Fallout titles forever. They need to develop a new title at some point.

Bethesda Game Studios has so far always only had one game in development at a time, which should be TES6 right now. If they are working on Fallout 5, we're likely talking pre-production stages. So, it might very well be the case that the two remasters come out in roughly equal spacing before Fallout 5 comes out in a few years.