Earlier quoted context omitted.
The criteria for calling a RNG "cryptographically secure" are incompatible with the game design goals here. The game needs a RNG that's stable when seeded, for reproducible runs. I look for the same kind of qualities when doing generative art. In comparison, a CSPRNG should be safe from oracle attacks, which is essentially the opposite goal.
I would expect all RNG algorithms to be deterministic and stable with their seed, but the cryptographically secure ones to have some additional properties like making it unfeasible to reverse the seed from the output, having a very long period or strong guarantees on the distribution of the output. It's just that using a 'secure' algorithm is often overkill for a game when you don't really need those extra guarantees…
It's not overkill here, because this is a turn-based game. The additional cost of using high-quality randomness instead of World's Worst "Randomness" will never even be noticed; I would have expected a CSPRNG to be the default approach.
That said, it is true that while the C# standard library provides a CSPRNG alongside its terrible PRNG, it won't let you seed the CSPRNG. So even if you know what the right thing to do is, Microsoft doesn't want you to do it.