And the big problem apart from it only being 2 cities was the content and how they were placed within the story:
- Lestallum was small and there's barely anything to do there. Plot-wise, reaaaaally little: most I remember is Jared-related stuff, which as you can imagine isn't very exciting.
- Altissia had some plot relevance, but the city was lacking in meaningful content too and was awkwardly placed within your journey. You never get to care about it or anything that happens there.
Had the game let you explore, even in the same vein as these two, a couple more cities or towns, sure, it'd be more than fine. Like, XV wouldn't be in the same level as others in the series in this regard, but it'd actually feel nice to have that much variety.
But at the end of the day, it was just two pretty-looking places with absolutely very little to do and not much relevance for your journey (and the open world really lacked smaller settlements that weren't gas stations or random houses. Some smaller villages/towns like that one south of Lestallum would have gone a long way).
Compare Lestallum and Altissia to cities like Midgar, Alexandria, Lindblum, Rabanastre or Narshe: no town in the series has ever been huge (about a similar size as in XV, really), but the games have traditionally featured a bunch of them (enough to let the player be immersed in more than just two different cultures) and given them things to do both for the plot and optionally. They feel, at times, as hubs of sorts, them having arcs in a way since you might return and things might be different.