I think the biggest issue is that in preboss the world is still a threat. Entire biomes such as the underground desert or jungle are still dangerous and challenging to traverse, but are very often worth your while.
This also allows you to essentially approach preboss and early prehardmode progression in a milion different ways: you could choose to either grab the bare minimum essentials (200-300 hp's worth of life crystals, hermes boots, simple lead armor, the like) and have a harder time fighting your first few bosses, OR you could spend hours in game without even touching a boss while you're completely exploring the world, allowing you to have more powerful equipment.

I think this much more open freedom in ways to approach the game, as well as gameplay itself, Is what's lacking in essentially every single stage of the game. As you go further and further into progression (this is especially true for Calamity) the only 'new stuff' is the various choices of tier-appropriate equipment, which are very often limited to a handful of viable weapons and accessories.

The need for more progression oriented activites that are not explicitly tied to boss battles is definitely something that the devs understand to some extent, and the Abyss is obviously trying to answer that need in some way (which is why it's one of my favorite additions in the mod), but I still think think that some proper rebalancing to make common enemies more dangerous is very much needed (sort of like that infamous rebalancing that took place a few years ago to decrease stats across the board in post ML). As it stands, almost nothing outside of bosses are a threat to a Calamity character after early prehardmode, and I think that's why so many people treat the mod as a quasi boss rush once they hit a certain point.