It is not about gears being fragile. There is no equipment that is designed for one axle to be working against the other. Simple physics. If you are on hard surface especially you see the result that much faster. There are some allowances for any setup for tire wear but even then the base diameter stays the same and that is where the tractor does its work.
You can call it simple physics - I prefer to call it design parameters.
4WD tractors ARE designed for the front wheels (at the circumference) to turn a few percent faster than the rear.
This is a very desirable attribute.
Weights and loads are KNOWN, coefficients of friction between surface types are KNOWN.
It is POSSIBLE to design and manufacture for anticipated forces (& uses) in fact it is necessary to do so.
On hard surfaces the forces build up until the tire/road friction is overcome, typically quite suddenly - known colloquially as "wheel hop".
Tractors and other 4WD vehicles DO THIS and they DO SURVIVE - at least the adequately engineered ones do.
It is impractical for someone, for example moving snow, to shift out of 4WD every time they come out of a driveway and get back onto dry road to make a tight turn and go back in. Similar situations exist in the fields, though not so obvious and the changes in traction are not so extreme.
I have taken the occasional local "road trip" to neighbors' places and noticed at some point that the tractor is humming along in top gear and still in 4WD - no toast, no mechanical noises, no drama, no damage.
Again, adequately engineered tractor transmissions and axles CAN and DO take 4WD use on hard surfaces - though tire wear is likely accelerated.
I don't suggest or recommend this, but I don't live under a delusion that parts are going to suddenly fail "in no time".
I maintain that anything unable to take this is poorly engineered and/or manufactured - in a word "FRAGILE".