Late(ish) to the conversation, but from my experience...
Firstly, the game is called "Dwarf Fortress", it does rather tend to emphasise the bits with the dwarves and their 'fortress'. It might encourage some people to think that the Adventure Mode bit is an add-on. It's not even the most obvious selection, when you start (I think, it's been a while since I've paid attention). And even if you do start an Adventurer, getting killed off by the first wildlife/nightlife/annoyed citizen that you happen to enrage is pretty final. (Whereas you at least have a chance, in a new fort, of trying to recover a part-killed embarkation party with whatever remains after whatever natural/self-caused disaster you encountered.)
Also, the procedural generation of settlements was always some way behind the possibilities of player-generated settlements. Those "Dwarf Mountainhomes" that were stair-lined holes in the ground, leading down to a normally totally empty (or, occasionally, ampersand-occupied) warren of unfurnished rooms? Elven settlements were just "a place where elves seem to meet". Human settlements were developed (procedural buildings with occasional procedural shops, eventually they got procedural streets, beyond the procedural highways between settlements with procedural bridges, procedural fields surrounding them, then better procedural buildings to make procedural streets, procedural sewers and then actual fortifications with procedural curtain-walls) and there were also the towers with the goblins (good for getting shot at through the arrow-slots)... It is improving!
As is the scope of interactions. Adventuring has always been somewhat of a grind. As has Fortressing, to some degree, but the variations and therefore the apparent monotony are so much more identifiable, compared with travelling (over a gloriously diverse landscape, it must be said) fighting (a wide array of) enemies/prey/unfortunates-just-crossing-your-path and having limited-script conversations (definitely improved, these days) with NPCs who you haven't decided it worthwhile killing yet.
But the adventurer still doesn't easily exhibit development. Stats/skills, inventory contents and a quest-list best denote the adventurer's advancement, but none of that is actually as 'tangible' as a Fortress, whether or not there's even an exhibitable megaproject attached to it. Which is not to say that an Adventuring Megaproject isn't impressive (the most obvious ones being either the Ultimate Pacifist or Ultimate Genocide/Omnicide aims), but... it doesn't produce the same kind of visuals.
Now... if as Adventurer you could actually build your own Empire... including literally building it (at first constructing your own outpost, from which to rule, eventually giving architects plans for them to create your castle and forts along the borders of your influence for your own (non-local) followers to occupy... or just direct a castle or fort to be built, to certain less-than-utterly-comprehensive instructions, and be able to trust to the engine/builder-AIs to give you something worthwhile... (Some of the early bits of this have been planned-for for quite a while, but I don't keep an eye on the game-goals enough to know how conceptually far Toady's actually thought of taking this.)
At the very least, we could benefit from a Battlezone-esque first-person-perspective "point and command" resource, building and unit management system... But that's a twenty-year-old game concept that I've never actually seen replicated in anything more recent, so I'm not sure if it was appreciated as much I think it ought to have been...