Probably has to due with the fact that the equipment that is required is built to be way more reliable, have to do with the corrosion aspect of salt in water, and probably that there tend to be more pieces involved with a saltwater filter vs. a freshwater filter to handle other aspects.

Have you tried looking on eBay or OfferUp or even on here for some type of used tanks in your area or equipment?

salt tends to eat at metal. So things need to have a special type of metal in order to not break down super early

Higher need for robust materials and precision equipment.