Because, you see - it was said to Keltham - returning to the Hypothetical Newly-Created Educated People, at some point their system is going to grow large enough that even with everybody receiving benefits for their participation in the system, there will still be defectors.  There will be people who just don't want to go along with the system, and try to eat food with a tag on it that points to somebody else.

Now the nascent Civilization needs police that can outfight any individual thief; and, since Newly-Created Educated People aren't stupid, they know they obviously need to ensure that the collective of all of them can always outfight the police.  Neither of these 'features' are cheap, and neither easily lend themselves to private ownership -

Because, you see - it was said to tiny Keltham - returning to the Hypothetical Newly-Created Educated People, at some point their system is going to grow large enough that even with everybody receiving benefits for their participation in the system, there will still be defectors.  There will be people who just don't want to go along with the system, and try to eat food with a tag on it that points to somebody else.

Now the nascent Civilization needs police that can outfight any individual thief; and, since Newly-Created Educated People aren't stupid, they know they obviously need to ensure that the collective of all of them can always outfight the police.  Neither of these 'features' are cheap, and neither easily lend themselves to private ownership -