How? That logic does not apply to Mikel who only just got back into the side, it doesn't apply to Matic who has been dropped many times this season, it doesn't apply to Cahill, it doesn't apply to Hazard who is a lot more likely to want to leave, it doesn't apply to Costa who is rumored to want to go back to Atletico, it doesn't apply to Remy who isn't getting any minutes, nor Falcao, nor Baba....etc.
Oscar falls in the 'Goldilocks' region of being a regular starter here while at the same time not attracting any major interest from clubs who can actually afford him.