I presume we're talking about the first game, to which I would say that you're right about the dungeons and lore contributing to the interest in DS1's bosses. Albeit, it isn't quite up to the scale of Demon's Souls level design and thematic approach when it comes to telling a story. When it comes to DS1's baseline plot, however, it's seemingly straightforward but it really isn't. The endings contribute to that, as two equally bad options (cemented as fact in the later games) just complicate your overall objectives. Learning that you really aren't that special and that it could have been literally anyone also complicates the matter (and that it was someone, just in an alternative universe, le flow of time). That's then followed by the important aspects of the story that are flat out poorly explained or just not mentioned at all (the crow).
This then brings us to the rest of the game's lore. Which at times is pretty interesting worldbuilding. But the worldbuilding is better for establishing atmosphere and immersion than it is at heightening the plotline of the game. Learning that Eingyi is the egg sac man and has a history as a pyromancer is interesting but plays no real part in the main plot.
All that said, you are completely right about ER. I started the Souls series again for old time's sake and it reminds me of how much it and Bloodborne both drew me in in a way that ER completely failed to do. I hate how easy it is to say why that is too.