It’s “the same” in a really broad hand-wavy sort of way. It’s not the same practically speaking.

FOSS usually does not have warrantees, SLAs, or developers in particular locations with particular credentials. The government has processes that they must follow for paying contractors, so “just pay them” is not something that can easily be done last minute, (and sometimes, not at all without a literal act of Congress.) Also the financial and obligatory relationship between them and the developer is much different, and must be managed differently.

Really, it is very important for people doing technology projects at the government to understand the difference between calling up the developer of Windows and calling up the developer of Git. Those are two very different relationships.

It's not so simple. The developers are not necessarily available for this kind of support. For example, I maintain an FOSS library, but I already have a full time job, and I'm not interested in working more hours. Unless someone hires me to work full time on this library, there is no chance that I provide paid support. I suspect that many other maintainers are in the same situation.