I don't think there's any evidence that those obstacles are surmountable, unless it's something like the Pope telling Michaelangelo to paint a ceiling. A bridge has defined scope and budget (ish) and a defined benefit attached to it, which many people will sign off on before it is commissioned, and it might take years to do, but it will also serve the local population for potentially hundreds of years in a practical way.
Actually, you provided an example where the obstacle was somehow surmounted [1].
The expectation doesn't have to be too specific or unrealistic. If you agree on some common ground [2], everything else can be fair game for the artist.
Your analogy with the bridge would apply if art also had a minimum viable version. Collapsed to its functional requirements, you could say that visual art is something to look at. But I doubt either party, especially the funding body or the public, would be happy without inserting some quality requirements (i.e., what makes something nice to look at).
Many artists do commissions, so you can see this as a commission with deliberately underspecified requirements.
[1] I won't get into the disagreements between the Pope and Michelangelo, and it's certainly not an example of a good contract, but we can assume that both parties were somewhat satisfied in the end.
[2] For example, both parties need to like it. Or the patron doesn't have to like it, but it needs to appeal to some public audience.
