I have a business involving catering and I know exactly why the club franchises the whole thing out. Firstly, it is the operator and not the club who has to comply with all food and hygiene regulations. This means keeping a record of the temperature of food sold, formulating a food handling policy and many other pieces of legislation. It means that the employment costs, holidays, pensions are the responsibility of the operator. Yes an 18 year old, employed for 2 hours per week will soon be entitled to a pension. It means that stock, stock rotation, VAT and other items are the responsibility of the operator and not the club.
The club, to run it themselves would have to employ a catering manager and have him administer all of the above. All for 20 minutes a fortnight, for nine months of the year, to sell mainly perishable food. I think the club is wise to avoid all that.