So why can't Apple fix this?
I suspect it's 'won't' rather than 'can't', because it may not be Apple's puppy. If Safari Cookies is the culprit, its
developer is the primary agency responsible. This is also assuming that SC didn't somehow expose some issue(s) in Safari's code Apple might want to adjust regardless of who's to blame in this particular instance.
Btw, the problem could still be a memory leak, but if so one caused by Safari Cookies instead of by Safari.
That calls for details...
Safari Cookies was looking for a file, ~/Library/Safari/Databases/database.db (I believe) that doesn't exist on my deuced Mac(hina), was not finding it, and was generating an error message every few seconds. (I found no less than 20,000 of them in Console, and there were more that I couldn't access readily.)
Now I'll extrapolate from an experience I had with Safari last night - a page wasn't loading, and Safari was beach-balling and its RAM usage (per Activity Monitor) was sky-rocketing - and guess that, although it was not beach-balling, Safari was consuming RAM as it generated error messages.
My understanding is that that is not
a memory leak in the true sense of the term but may be one in one of its corollary senses.