After all they're not made to wet sump when coming from the factory ,so what's going wrong
We are.
From new they were designed to be used as daily transport, that meant firing it up at least twice every day if not 4 or 6 times.
When used this way very few wet sump, clutches don't stick & carb slides don't gall .
When used every second Sunday, only if it is a fine day lack of use problems manifest themselves.
Best solution is to fit a sump with a magnetic drain plug.
Every time you get home, before you even take your hat off, drain the oil from both the tank & sump.
Put the plugs on the tank to remind you that the bike has no oil.
The oil will be clean enough to be collected and put into your 4 wheeler and the oil in your engine will always be nice and fresh so you will end up getting 2 to 3 times the life out of your bottom end for the cost of about £ 200/ pa as comparred to £ 1000 + for a rebuild.