Some of the "reasons" are:
1. yes some people are not happy with thier position at Virgin and choose to leave and go onto other careers
2. Virgin encourages internal promotion therefore some crew move onto other areas within the company
3. When you have a rather high number of females within the 18-35 region you are going to expect a few crew to be on maternity leave.
4. expansion-- for every aircraft we get we need to put through both a training group of initial cabin crew (25-35 people) and a training group of cabin supervisors (7-12 crew)
5. then of this crew you will have quite a few on holiday leave and sick leave, so we need numbers to cover that.
I'm sure that there are more "reasons" that we are always recruiting, but just remember that HR only take the applicants that they consider meet the criteria. They may take 10 from one group interview and none from the next.