The problem in Keynes thinking on "social services" is that it's impossible to gather all of the knowledge necessary to ascertain what those would be. That knowledge is dispersed among ~320m citizens (in the U.S.) who each have particular wants and needs. And even if you could somehow attain that knowledge in one place, the likelihood of finding consensus on how a particular "social service" should be offered is next…
Why don't we just let individuals choose if they want to have car insurance? If they want it, they can have it - if they don't, they don't have to. The same (very good) reason we don't allow that is the reason we don't allow this: a lot of individual decisions have social costs.
We do allow them to choose that: you don't have to have car insurance, not even if you own a car.
We don't, however, allow them to drive on public roads without liability insurance or (in some jurisdictions, like CA) posting a liability bond.