I don’t know either but my bet in these kinds of situations is usually historical reasons. At some point, these contributions were added and it was probably an easier bill to pass by splitting the amount in two.
As for the negociation part, it’s not really negociation but when I worked as a waiter (in several different places), I’ve very, very seldom got anything above the minimal conventional wage. My take is that in those jobs, salaries wouldn’t necessarily be higher if the employee paid the whole contributions, they’d just have less to take home. It’s hard to say there’s a lower limit beyond which the jobs wouldn’t be taken because it’s the kind of jobs where workers from other countries with lower costs of living compete so permanent residents may be simply driven out of the market and into unemployment insurance or other social services (providing, once again, indirect subsidies for business by having the community pay for the low wages).
The governement is the only insitution that can guarantee that the pension will be there when you get old, unless we collectively fail as a society (though it’s on us to work toward prosperity so that we can flourish as a society). Even pension funds can fail and have the money you’ve put in disappear.
That’s what I often end up thinking when I compare the U.S. societal system and ours, on a different level: we tend to be heavily protected against ourselves. From what I understand of the U.S., it’s a much more predatory society where they easily let the sardines sit at a poker table with the sharks. Not saying personal finance isn’t an untouched subject intentionally, here, because I do think so. We are a land of insurances, some serve a real purpose but many flourish on our ignorance. And they pay for part of our taxes and pensions so I’d wager some people think it’s actually for our own good that we are uneducated.
Thankfully, internet and widely available means of communication are providing means to somehow change that. At least it’s evening the odds somewhat, giving people grown up in poorly informed background a shot at getting their fair chance at the table.