KDF for generating RSA and ECC keys from a 2048 bit secret Do KDFs exist for generating RSA and ECC keys from a 2048-bit random secret?
Rather no. There is no standard construction going by the name Key Derivation Function aimed at generating public/private key pairs. Traditionally, especially for RSA, how to generate a key pair is left at the discretion of the implementation, rather than made per a specific deterministic process. The usual name is Key Generation. And it's usually stated as a randomized algorithm, rather than as a function.
For ECC per Sec1, it is straightforward to define such KDF. The private key is an integer $d$ in range $[1,n-1]$ where $n$ is the fixed, public order of a generator on a curve. Thus we could take the 2048-bit secret¹ $s$, and turn it to the private key by applying $d\gets(s\bmod(n-1))+1$; then compute the public key by applying $Q\gets d\,G$ using point multiplication. Since in practice $n\le2^{800}$, the bias on $d$ is negligible if the 2048-bit $s$ is uniformly random.
For RSA, that's less easy. We need to specify a Cryptographically Secure Pseudo-Random Number Generator, seed it with the 2048-bit secret¹ $s$, and generate the RSA key per some precisely agreed-upon process. FIPS 186-4 appendix B.3.2 is next to being precise enough. Also, beware that it is hard to secure such computations from side-channel and fault attacks.

¹ Or a derivative $s$ of that secret by a symmetric KDF.