You have been down voted, but the question is reasonable.
One reason is so that invocations of ssh outside of the context of user invocation of ssh at the command line will also have these customizations included. This is especially important for ssh, which has emerged as a main security interoperability tool for Unix systems.
For example, if you use rsync, the tunneling and host alias conventions you set up in .ssh/ will carry over transparently to the ssh tunnel used by rsync.
Another example would be invocations of ssh in scripts (sh/bash scripts, even) that will not or might not read your .zshrc.
Ya, I think the underlying issue is I do things very differently than people on HN.
The idea of creating dependencies on a configuration profile inside a bash script is the exact opposite of what I would do.
I also could not rsync things to my local machine [bandwidth constraints] and would be rsyncing between remote machines, which being a shared environment, I would rely on explicit invocations instead of creating configurations/aliases.
Thank you for telling me how/why other people make different choices. I always do seem to have the blinders of my process is the only process I consider when commenting on HN. :)
To add to what @mturmon said; you'll want to keep in mind that even GUI tools (i.e. consider your favorite database tool that supports SSH connection) that support SSH connection will pick up this configuration so you don't have to manually plug in all of the pieces for each session.
Just specify the alias host name as configured in ~/.ssh/config and the user, identify file, and anything else you put there will be used as set.
Maybe it is just me, but I prefer to dump all my custom commands and aliases into .zshrc so they are easy to backup/track/find.