2048 bit hashing and 1024 bit symmetric encryption
Because the numbers required to achieve that level of security are much lower, even versus an adversary with a quantum computer. See this question/answer on security.stackexchange for the specifics. 

Assuming that 256 bits/512 bits are secure for encryption/hashing, anything greater cannot serve a functional purpose in terms of security (it cannot be "more secure"). 

The biggest major factor besides security is performance. If the algorithms are not more efficient then their traditionally sized counterparts, then there is no motivation for their creation. Your proposed extremely large algorithms could potentially have performance issues. Using copious amounts of data is not necessarily register or cache friendly. Such large algorithms could also be difficult/expensive to implement on constrained/dedicated hardware.