"Just because you have to operate your own gear, flaps and comms hardly make it enough to need a completely different renewal every year."
No gear on most SEPs but you do have to operate your own flaps and comms.
Of course that is not the issue. You also might have few or any automatics, and no one to organise the plates for you and cross check what you are doing - it is potentially all hand flying with the basic legal minimium instruments and little redundancy. How are your single pilot partial panel skills?
I suspect not many commercial pilots without recent SEP IR time would want to operate in these conditions without some recent currency.