Are all 64-bit installers of Linux distros have command line installation setups? If so, why is this the case? If not, then why are most 64-bit Linux distros have command-line installation setups? Are there existing 64-bit installations that are GUI?
I'm not sure why you got this impression. All of the major modern distributions include GUI-mode installation for both 64-bit and 32-bit versions.
Here's the 64-bit DVD ISO for Fedora 14, currently the latest release:
And here's the same for Ubuntu 10.10, also currently the latest:
I know for a fact that SLES 11 has a GUI installer, and that comes in 64-bit flavors. I see that RHEL 6 also has a GUI installer (link).
All of the RHEL-based installations (including RedHat linux itself, and derivatives such as CentOS) have GUI installers for both 64 bit and 32 bit systems. In general, I believe most major distributions use effectively the same installers for both architectures.