Is a stream cipher and a (cryptographically secure) PRNG nearly the same thing?
Stream cipher and CSPRNG aren't the same thing. CSPRNGs must handle initial seeding, post-compromise reseeding, back-tracing resistance among other things; stream ciphers only need to be able to output something indistinguishable from random.
So can we turn a CSPRNG into a stream cipher? Possibly, if we make every generate requests to the CSPRNG idempotent, and drop reseeding. But that wouldn't be as efficient as just using the stream cipher directly - not only does CSPRNGs has extra code paths to execute, its internal state is often larger than that of stream ciphers.