It’s difficult to interview while working full-time. Scheduling is a nightmare, studying is tedious, and it feels dishonest hiding from your colleagues. Recruiters only want to talk during bank hours, but you still have a full calendar of meetings and a full week of tickets to complete.
Unless you have a very good relationship with your supervisor, you don’t want to make your job search too obvious until much later in the process. Keeping a secret from your peers is easy, but keeping a secret from someone whose job is to make you more productive can be impossible.