Precisely. The simple fact is that if you close down what is effectively an entire land worth of staff, including front gate, attraction operation, food and bev, and custodial, you've gotta take a hit - either a financial one by redeploying them elsewhere, or a recruitment one by letting them go and hoping they come back next season.
Let's be real - The April holiday period is still warm enough to swim, as is the september period. If you're going to close down, you're only shut from May to August - just 4 months. It's too long for your staff to survive, and they'll 'find work elsewhere', and then if they do - would they come back, knowing they'll have to do it all again next year?
I know WnWS had return staff - @StingRay is a testament to that - but its a job, not something that you'd sustain long term.
This means that for the next season, you have to recruit. And four months to recruit staff for an entire park is about what is needed for application periods, group interviews, training etc... and there's a lot of lost knowledge as your new staff are going to be relearning the lessons last season's staff already knew... so basically you're advertising for next season the day your staff finish up for last.
Most staff are casual, so simply cutting back shifts in the winter time (when it's to be expected anyway) is a far better way to keep most of your workforce on board for the following season - instead of 6 shifts per week, they get 3. They might supplement another job in the winter to boost their income, but they're still 'connected' to the park for the next year.