Actually they get cleaned quiet often, but depending on where they operate from and can become dirty quite quickly, problems you have is you need somewhere with a proper wash bay and drain traps to do them and it is a bit labour intensive, I agree, I have seen some really dirty ones about and they have gone out of maintnance looking the same, it does them no good if a customer sees them really bad as it makes them appear uncared for, they then wonder about the rest of ther maintenance, and quite rightly too..... You have to remember these things are flying literally around the clock and to drag it off fare paying flights is a none starter, so they tend to get done during maintanance cycles and washed before they take them apart.
Most corporate ones have companies that come in and do them, or their own staff to keep them looking pristine.