Look, at the end of the day, you're speaking from the experience of a core player, and I'm speaking from the experience of a leader (and not one like flogger that struggles to grow/login/play their prov). There are plenty of war kd players who are absolutely high quality and would be great core players, given instruction and teaching on how to prep, how to whore, etc. However, as a leader, I would rather have a core where I don't have to worry about more than a small subset of players where I've explicitly decided to accept the risk of needing to micromanage them. We take in war kd players all the time when we need fresh blood -- they're some of the most active players, which is great. But they're not the kind of player you'd trust in their first age with a meaningful role.
Some kds (like pandas) like to have their races split and specific players for every kind of core role. CR generally prefers to have as close to uniform-race cores (or duo-form with a hybrid + a hitter) as possible. Particularly when running a uniform core, you want to be able to double the player that is performing the best, without wondering whether that player will be a competent calf now that you've put them up -- because that's what gets you the best outcomes. If Meep is your largest core prov, you want to be able to turn them into a calf (less pool used than doing someone smaller) without wondering if Meep will be able to pump effectively. The more you dillute the talent pool of your kd with unknown entities, the more risk you take if you do that.
For a leader in most kds, you don't want to take on unnecessary risk, and ultimately that's what bringing in random war kd players means unless you've already established a relationship (or they have relationships with folks already in kd) and you're confident of their ability to get on level.
e: fair point PC. I do feel like it's a little more interchangeable in war tier, but you're right that the number of 'just follow orders' core is still not like 20 players.