They leave long gaps so more go; means more fees (for them);
Of all the seminars I've attended, they've said the breakeven point (at about £250 per person) is about 20 attendees. There are only a finite number of people needing to attend a seminar, so it's not cost effective to run them every month with only a few people turning up.
I think On track and Aopa run 3 a year each, so thats 18 opportunities over the validity of the rating to get signed off. remember you can attend a seminar at
any time during the validity of the rating to meet the requirements for revalidation next time round