It's a combo of all those things: inflation, improving the cash flow cycle, and even internal rate of return (i.e. for those 60 days the money is hopefully being used in a capital efficient manner).
I'd say it's mostly because "that's how it's always been", and because they can.
That's how it's always been, because before computers, it typically took 7-10 days to process through company procedures and banks.
And the cash flow cycle / rate of return is laughable in today's ZIRP environment. Especially if you are bigcorp, you can pay at Net+0 and take a loan at ~0% (+/- 0.1%) yearly, which translates to ~0.02% over 2 months, if you so wanted -- and no one would notice.
When I did contracting, I would happily give a 5% discount to get Net+0 -- in fact, a couple of times I raised my rate 10% a-priori and offered a 10% discount if they paid Net+0 instead of Net+30 (the latest I was willing to accept). None eventually accepted; One tried to get it through their bean counters and couldn't - the Net+X for X>30 is so deeply ingrained into most accounting departments that it can be considered an axiom, regardless of how little financial sense it might make in a given case.
