ssh-agent outputs the environment variables you need to have to connect to it:
SSH_AUTH_SOCK=/tmp/ssh-492P67qzMeGA/agent.7948; export SSH_AUTH_SOCK;
SSH_AGENT_PID=7949; export SSH_AGENT_PID;
echo Agent pid 7949;
eval you immediately load those variables into your environment.
As to why
ssh-agent can't do that itself... Note the word choice. Not "won't", "can't". In Unix, a process can only modify its own environment variables, and pass them on to children. It can not modify its parent process' environment because the system won't allow it. This is pretty basic security design.
You could get around the
eval by using
ssh-agent utility where
utility is your login shell, your window manager or whatever other thing needs to have the SSH environment variables set. This is also mentioned in the manual.