I can remember driving vehicles that required double declutching for gear changes ..... The problem is that the FMC manufacturers still believe that the pilot can pick up the bag of bolts when their software fails and seamlessly go from fully automatic back to double declutching.
Therein lies the difference .. the synchro/auto gearbox doesn't fail back to a Road Ranger box reversionary mode. The aircraft, on the other hand, does .. ergo, if the pilot is not up to speed on the day then it all turns to custard on occasion.
I have a non-synchro truck licence .. my proficiency probably isn't pretty .. but, if necessary, I can drive one. Same philosophy.. why let a skill (which may have a value) atrophy ? ... with a sideline consideration that risk probabilities should drive the decision to a large extent .. ergo, the truck thing isn't all that important .. while the aircraft AFCS failure still is until, and unless, systems reliability can get up amongst the structural failure probabilities ..
Yes, it costs in money and time .. as safetypee observes, it's largely to do with risk management and desired outcomes. I might look back to yesteryear wistfully but I was only too happy to do an autoland at the end of a long tour in lousy weather conditions ....