Why aren't stream ciphers extremely secure? The way I understand it, stream ciphers are pretty much one time pads off-sourced to a PRNG (the seed is the key that generates the fake one time pad).

So why not just use one or more CSPRNGs (concatenated seeds, xored pad streams)?

Wouldn't breaking such a stream cipher be equivalent to breaking every single CSPRNG used?

Yet we have things like RC4 riddled with vulnerabilities, is it because performance is a concern? There are many applications where it's not, why not have something like I described for those cases?
Yes, stream cipher is to OTP what CSPRNG is to true randomness.

Yes, breaking a stream cipher implemented with CSPRNGs XORed together (each independently seeded) demonstrably implies breaking each CSPRNG (or a side channel).

Yes, there has been less than perfect stream ciphers/CSPRNGs around. I guess that's for a large part because of efficiency concerns, and because their security was adequate for many of the intended uses (RC4 was designed with performance in mind, like 30 years ago; the first commercial use I saw of it was real-time decryption of Adobe Type Library fonts distributed encrypted on freeby CD-ROMs, with each font category unlocked by a 40-bit key).

I guess that we nowadays use single-CSPRNG stream cipher (like AES-CTR, Salsa, ChaCha..) because simplicity and speed are virtues. They are believed extremely secure, except for implementation attacks (penetration of platform, side channel).