why can't anyone make a fully enclosed 10-speed bike chain/gears?

I agree, not really for guys that actually bounce over logs and big rocks. I've done some off road biking but just puttering along, but through pretty clingy dirt.

Motorcycle with "covered chain"??? You mean just one of the top covers like many bicycles?

I'm talking about a full top and bottom cover like the Huffy patent.

IIRC those top covers are "guards" and more to keep your clothes from getting caught or dirty from contact with the dirty chain, not to keep the chain clean.

Any bike mechanic will tell you those old style full chain covers do a great job keeping the chain and gears clean for months of riding in wet dirty streets.

IIRC one British model included a wicking brush that rode on the chain and reservoir of lube. These bike's chains were said to have lasted decades on dusty roads in "the colonies" where replacement parts were hard to come by.

 

Belt drive with greater flatness profile can trap dirt, mud, pebbles, twigs, etc. between the toothed belt and sprocket gear teeth. Sometimes pieces of this debris might remain embedded on the belt surface, affecting proper tension, and making pedaling wobbly/ difficult or possibly derailing the belt. At least that's what's happened with kicked up soil on my garden cultivator uncovered belts. Maybe a beaded chain cable drive + gearhub would work, if it could be made strong enough and didn't need to be lubed?

PC

 

A commuting bike is slower and heavier for a reason. The bike is used in all weather situations, it has to withstand abuse and neglect, falls on cobblestones, scratches, dings, it has to carry the child on his seat or pull it along in a trailer, it has to transport the 40 pounds of groceries and so on, all that while the rider is riding comfortably in his office clothes. Weight and speed are secondary issues.
If the situation demands high speed commuting then a electric bike is the better choice. Or do you want to arrive at the office dripping with sweat? Only people without this problem use fixes or roadbikes for commuting.

The world advanced since the old 3 speed hubs from 1903 and 1904. The Rohloff Speedhub gives 526% overall gear range in steps of about 13.6% and 14 real gears. The Shimano Alfine 700 has 409% overall gear range and 11 speeds. The 7-8-9 gears models (Nexus, SRAM) give around 300% overall gear range.
The Rohloff is actually rare in commuter bikes (even high end) and mostly used for mountain and touring bikes. Touring means transcontinental expedition touring.
The 1-5% performance loss from a planetary gear is actually comparable to derailleur system losses. The added weight and greater expense is a price payed for convenience and ruggedness.

If you really want to do this flexible enclosure you need to resolve one problem, and that is the crooked chainline so that the chain does not rub against your enclosure in any gear combination.

 

A frame attached to the frame that holds the two little tenser cogs on the rear derailleur, with frame that comes up high enough to clear the biggest, innermost rear sprocket should be able to follow the chain through shifts.

Only problem is making sure it doesn't put too much force on the derailleur, at least in side to side motion.

But I think a thin wall plastic housing would work, too. Since the rider's feet maintain good clearance away from the whole derailleur at any setting, the housing can be Max Gear Movements + 5mm all around the drive-line and you'd never know it was there without looking.