This website uses a 2048-bit key. 512-bit RSA keys are considered generally insecure and easy to crack. 2048 is the industry standard. Your point is probably still valid, but we don't know how high we can go.

512 bit primes lead to an RSA key length of 1024 bits, since the key length is the length of the product of two primes.

It's not about finding a large prime, it's finding prime densities, gaps, sequences, etc