Recruitment is tailored to meet a need. If internal recruitment is sufficient then there is no need to take the risks attached to external recruitment.
Remember, an internal applicant will already have been given basic training, acquired skills above the basic level and more importantly been observed and reported on before getting to selection. The outcome for an internal recruit is more assured than for an external applicant.
It is also one reason why it seems harder for an internal recruit to gain a commission or become aircrew.