Plus, I can only imagine what would happen to the GOP/austerity/small government types if you started suggesting they move all those (necessary) function in house. Oh man that would be a fun debate to watch.
One is surge capacity for occasional needs. While the federal government is paying for some road building somewhere all the time, they might not want to hire permanent civil servants in a particular place for a project that's only going to last a few years. There's some sense to that.
Another category is where the government is buying goods or services off the shelf that lots of other entities buy too. That's your fuel example. Here again, there's sense to not duplicating effort.
But the last category is where the government is the only buyer and it's a permanent need. And that's where I don't see any sense to it.
Just to be clear on my point, it's not very hard to go and buy a portfolio of student loans for yourself (assuming you'd want them...)
As for the road example, road building is an inherently local activity. If the federal government hired someone to hold a flag in Topeka it's be pretty unreasonable to expect him to start holding a flag in Phoenix next. So in that sense the need is temporary. But loan servicing can be done anywhere. My servicer is in NH for example. So the government could hire a bunch of people anywhere and bring it in house.
Same thing with healthcare.
In mortgages, student loans, and healthcare, the government is already heavily involved on all aspects of the transaction. They might be providing insurance for a mortgage via FHA, student loan guarantees for a federal loan, or paying via Medicare. Private servicers/insurance are, in these cases, usually an unnecessary middleman that exists mainly to make a profit. In much of the rest of the developed world, the government acts as a single-payer for education and healthcare, and is more involved in housing than in the US.
Having the government take over supermarkets would be less efficient - it'd be adding complexity to the system, not reducing it.