I've worked in government as well as multiple companies (from startups to huge multinationals). So, far only the government was adamant about speccing the requirements up front, waterfall-style. The commercial enterprises, even including an old and large bank, are much more open to agility and "not knowing exactly where the road takes us". Also, like you said, the government is much more into buying software - either COTS products or something tailor made to a spec - than developing it. It's mostly because the govt just cannot hire and retain good enough developers. The commercial side of the world is fine with developing in house, and most large companies have at least hundreds of developers on payroll.
As for the waterfall in govt vs agile in commercial world, I think it's about difference in core principles - the govt is very much about accountability (being able to account for every dolar spend, not wasting taxpayers money no matter what), while businesses are ok with some waste as long as the end result is still worth it.
> That sounds like the government is all about accounting, not accountability.
It's sad but true. Many government projects happily spend 2X or more just to know exactly where all that wasted money is going.
This is likely a result of the fact that taxation is mandatory, whereas investing in a company is not. If you are forced to pay for something, you're more likely to scrutinize how that money is being spent. If it's optional, there's a much greater degree of trust.
It's more than just accounting - everything needs to be pre-planned, there's no spontaineity or thinking as you go allowed. It's probably closer to medical devices software that to a typical BigCo web/backend software.
As for the waterfall in govt vs agile in commercial world, I think it's about difference in core principles - the govt is very much about accountability (being able to account for every dolar spend, not wasting taxpayers money no matter what), while businesses are ok with some waste as long as the end result is still worth it.