It depends on the employer but I find it quite common. Non-technical hiring managers might see a full-stack developer as a both a backend and frontend developer and it seems very attractive. Top tier employers usually want the best of the best and can afford to have multiple members on a team and thus allow that specialization.
If you specialize in a certain skillset, focusing on that skillset gives you time to be even better. Sure, there's developers that can write both back-end and front-end code but they've given up a level of specialization in order to do that. It's not a reluctance to learn but a different focus of what to learn. It has nothing to do with passion.
Most full-stack developers I've met are really backend developers that can also write some front-end code. They don't spend the time to learn all the intricacies of CSS/HTML or stay up to date with changes in spec / new tools and methodologies. I don't know you at all so I can only guess but if you think about it, do you find yourself better at either the frontend or the backend? It's very rare for someone to be a top 10% backend developer and a top 10% frontend developer. I could call myself a full-stack developer because I'm capable of writing both portions of an application but I know that I don't write one end as efficiently or securely as a specialist.
If you specialize in a certain skillset, focusing on that skillset gives you time to be even better. Sure, there's developers that can write both back-end and front-end code but they've given up a level of specialization in order to do that. It's not a reluctance to learn but a different focus of what to learn. It has nothing to do with passion.
Most full-stack developers I've met are really backend developers that can also write some front-end code. They don't spend the time to learn all the intricacies of CSS/HTML or stay up to date with changes in spec / new tools and methodologies. I don't know you at all so I can only guess but if you think about it, do you find yourself better at either the frontend or the backend? It's very rare for someone to be a top 10% backend developer and a top 10% frontend developer. I could call myself a full-stack developer because I'm capable of writing both portions of an application but I know that I don't write one end as efficiently or securely as a specialist.