I'm a player who usually plays the same character, and mostly it's just that I don't feel confident with new characters. I'm always worried I missed something big in some optional novel or something, or that I just can't write their voice right, so I usually fall back on the one or two characters I know I can nail the voice for. I also don't get tired of it because I'm usually apping to very different games, where my character would interact and develop in very different ways each time.

Of course, I can't speak for whoever you're talking about, but it's not necessarily highlandering or soulbonding or anything weird like that. Probably just a matter of what they enjoy / what they're comfortable with.