What about carers? Fast food staff? Retail staff who help you choose purchases? I know you don't tip those because no-one does. Yet they do very similar work to all the people you do tip, for very similar money. Makes no sense.
DP used to work in a high end bike shop where he'd regularly spend 30-60 mins helping customers choose a custom bike, including fitting saddles, adjusting a bike so they could go on a test ride, building the bike, advising on accessories etc etc. People were spending thousands on these bikes. He got paid NMW.
A couple of times a year a customer might bring a crate of beer in for the staff but apart from that, not one person ever tipped.
Yet people somehow think that someone pouring you a drink or carrying a plate of food over to you, for which you've already paid handsomely for and the person doing the job earns at least as much as those who never get tipped deserves an extra reward? I don't get it at all.
OP, what are the 'such long hours' that spa staff work? It's unlikely to be any worse than anywhere else/standard full time and the terms and conditions look quite good for hospitality on their recruitment page, above minimum holiday allowance, free day passes for Center Parcs, sick pay, heavily discounted holidays, profit share scheme.
I'm not arguing for a race to the bottom or think people in these jobs don't deserve recognition, but as jobs go, it's probably about as pleasant and stress free as you can get compared with other ways to earn that sort of money.