To be fair, with a nice 100% wool jumper, you don't really need to wash them that often. A gentle wash once a year or so before you store it for summer, and spot clean to get rid of surface dirt when it appears. You're not wearing it next to the skin (or you shouldn't be), so you shouldn't have issues with bodily grease and sweat.There’s nothing quite like a sour ‘pit smell in a greasy jumper
Anything at all synthetic though will hang onto pongs like a mofo, and should be washed regularly. Wool will just smell like... Wool, if you air it properly. Even cooking smells come out on their own (tested after being set aside for a bit because I couldn't be arsed to wash it after having it in the kitchen while cooking and it smelt a bit.)