Why? They could just integrate all the DLC into the main game and then use that as the foundation of a sequel, all the development is already there and done. There is no reason to assume devs have to start over from square one with every entry and slowly build towards not being shit. When games were released on CDs and DVDs they were usually good from day one, like the first Victoria still is, and sequels were expected to improve upon what came before since the work was already done there in terms of systems and design. That's what a video game sequel is supposed to be, a new and improved version of X with new content, unlike expansions which are just new content.
There are several problems with your proposal. One, it would mean that game technical side remains outdated. There's only no much that can be done to prop up old engine, and V2/CK2 ran on functionally other engine at this point than their sequels. Two, every DLC beyond initial projects has a problem of ruining game's tight initial design, which can be clearly seen in Ck2 - IMO Artifacts, Secret Societies and Bloodlines push the bloat beyond acceptable level. Three, if the game is is just "previous game plus", there's less economic incentives for new customers to buy it. Four, what if some systems clearly don't work and need total replacement? When they are tightly integrated into the systems (like EU's trade, or Vic's warfare), dismantling them requires a revamp of the entire design.
And frankly, I don't want Paradox to just churn out the same 'improved' (bloated) product over and over, I want them to take risks and innovate, even if they sometimes fail. If I prefer older solution to the newer, I can always go back to the CK2 or Vic2 - like I did with HoI when fourth entry, which I despise, has came out.
If, for example, they didn't innovate with EU4 and left the primitive trade centers system from original EU1 in place, the game never would've evolved beyond a board game, as the new solution introduces better, more 'simulationist' and sophisticated dynamics. This in turn positively influences not just economy, but also naval warfare, colonisation and the entire strategic outlook of the player. Same with V3 - I might not like warfare in this game, but to me it's clear that without completely revamping military systems of V2 (and by extension, all connected systems - so... half of the game), the series would not have been able to progress.
