Tobias didn't exactly have a personal life (due to being trapped in morph) so I guess that made it seem like the only time we saw him was on missions or doing something connected to the war. He didn't really have a family anyway and spent a lot of time on his own or hanging out with Ax when they weren't on a mission, so we didn't really see him unwind after a mission like we saw with the other human Animorphs. Ax and Tobias didn't have a home to go to, but Ax talked about Andalite things a lot so maybe that's why you feel like Ax was less of an enigma.
Although I understand what you're saying, I think I identified with Tobias quite well. To me, all the Animorphs had a part of their personality that I identified with, which is one of the reasons I liked the characters so much.