Because 4096 bit RSA is a lot slower and bigger and those things matter. And there isn't any upside? If you're actually worried about 2048-bit RSA (and you should not be), you should switch to one of the elliptic curve schemes.
I'm not actually sure about this. the elliptic curve schemes are just as broken with quantum computers, and the larger key size of rsa seems like it might add a few years of overhead in terms of qbits needed. not an expert though