oil filters get changed a lot so it is profitable for aftermarket companies to make them and for owners to use them.
Hydro filters might get changed every 5 years ( if then) so the demand for them is not high enough to bother making an aftermarket filter.
Hydraulic filters are really tricky cause flow rate is very imports as is back pressure & bypass pressure, let alone the micron rating of the media used.
Stens , oregon & Rotary market a range of hydro filters, usually from the OEM but you can not get them from a Scag dealer as the franchise agreements usually prohibit fitting aftermarket parts and reps are really good at spotting them on the shelves when they call by.
A franchise is worth a lot more to the owner than $ 2.50 extra profit on a filter they only sell 1/2 dozen of a season.