1. KMail, unlike Thunderbird, does not have a spam detection engine built in.  It requires the system administrator to install one for the users.  When you tell a fresh KMail to apply spam filters to inbox, it responds with a disappointing message that no external spam detection engine has been found.  This would cause any lay user to turn away from KMail and use Thunderbird instead.

2. Dealing with spam is hard and requires software support, including on the client side, especially for users who use free mailboxes that spam their clients as allowed by their terms of use.

3. Of the spam detection engines recommended by KMail, bogofilter seems to be easiest to use.

4. It is hard for a lay user to install bogofilter after the initial disappointment.  You cannot find it in Discover and you must escalate to do it.

Therefore I postulate as in the summary.