While better security is a goal of relibc it is not actually one of the primary goals.
Most existing libc implementations are not particularly portable across different kernels, and especially not to microkernels such as Redox OS. The goal is to have a standard library that is easier for the Redox OS devs to develop in (obviously, Rust makes sense here), and not so tied down to one kernel like musl is for example.
There are important reasons why Rust should continue to use glibc by default, but the Rust compiler already supports musl libc on Linux, so glibc is not required.
It’s a mechanism in glibc for loading dynamic libraries at runtime (based on a config file) to handle DNS and some other things. Anything statically linked can’t support this, which can cause issues with some setups.
Most existing libc implementations are not particularly portable across different kernels, and especially not to microkernels such as Redox OS. The goal is to have a standard library that is easier for the Redox OS devs to develop in (obviously, Rust makes sense here), and not so tied down to one kernel like musl is for example.