In regard to your question as to why the tubers are more stable in the ground (assuming an mild enough climate), I have no answer except to confirm your observation.
Maybe tubers that are never dug preclude the mistake of digging them before they are fully hardened?

I read with interest recently, in one of her gardening manuals, Nancy Ondra claimed that she lost a lot of the tubers she tried to store until she quit washing them and instead stored them still encased in the ball of garden soil. This technique combined for her the advantages of still being in the soil, but the non-freezing temps of a protected area. The root balls were in a open, plastic-lined container so she could add a cup of water once in a while if things seemed too dry.

im my zone tubers survive in the soil, but large parts of it rott. I had left three dull cultivars in the ground as a way of culling, and 2 sprouted the flowing season. When I prepared the bed and removed those bulbs, the clumps where mushy-messy-rotted and only a smallish part was growing.
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I think that in an average modern building air humidity and temperature are the problem or the key. The air is too dry to keep tubers all winter without covering stuff.

I store my tubers in a cellar/basement that get`s somewhat heated just by pipes and stuff.

I keep them in plastic boxes covered in something like turface (but no dusty particles)
basically little bits of backed clay.

So it is soil-less, well airated and the bits retain enough moisture to keep the bulbs from dehydrating. I check them regularly, like every second week and add some water, very carefully.

Sometimes the dry old stems get / show mildew, then it was too much water.

As we have wireworms (which eat their way into Dahlia and potatoe tubers and others) in the garden (larvaes of a black beatle), I really prefer to wash my tubers in order to get rid of all those critters.

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and about mature or hardened tubers.
This year I dug up my first batch of tubers while it was still warm (in order to stagger the work). Some of those got really squishy and soft after a few days.

The last batch got some days of frost (because I couldn`t work fast enough).

After washing them I just let them dry in the basement, not covered yet.

And those are still nice and firm after a week.

So, some aspects to consider,

bye, Lin

Assumptions are a reason many people fail in endeavors. Dahlias do rot in the ground and do not do better than when tuber clumps are properly dug and divided. Storing whole tuber clumps over the winter is less successful than divided clumps. Tubers store better. I store thousands of tubers and also store some whole clumps. A stored clump is more susceptible to rot from the stem and drying out. I divide many of these stored clumps in the Spring and find that I get 50% fewer tubers to plant. I have left many clumps in the ground and dug them after one or even two years. Varieties that make small or tubers that tend to rot, nearly always fail to survive into the second year. Those that make larger, easily stored tubers can and do survive in the ground if they do not freeze or get flooded. Freezing weather freezes tubers clumps stored in the ground. A thick mulch can help in some climates but in many cold climates, even mulch will not prevent freezing. Abnormal amounts of rain can kill dahlias any time of the year if the water pools around the plant for more than a day or two.

If the tubers can't reliably get through the Winter in your climate, I'm certainly glad you dig them, because I enjoy your dahlias very much.

I've noticed that, once dug up, a clump of tubers gets colonised by mould etc within 2 or 3 weeks, especially in a shed which, although having some ventilation, the weather is damp. As for dividing them, if only tubers could be divided as easily as a bunch of Bananas!

If they've been grown in a pot, you're looking at a tightly congested mass, having a mulititude of thin stems where you just don't know where to start.

It's not so bad when they've had the extra space of open ground, as the individual tubers are larger and spread out, and have a thicker, main stem. I think that the loose 'floppy' tubers, which have a very thin neck, can be detached, but others are bonded onto the main stem, which makes any knifework a daunting prospect.

Tubers grown early on in a pot are compressed and contorted. This includes especially, plants grown from rooted cuttings. I divide hundreds of such clumps each year and get an average about 3 viable sections. They may not look like tubers but each section has at least one viable eye and they store well. In the UK it is common practice to store clumps or partially divided clumps over the winter and then they are used to produce sprouts to make rooted cuttings. While many growers may not do this, the commercial sellers do so and then sell the rooted cuttings to the back yard gardener. The prices of such rooted cuttings is less than what we pay here in the USA for tubers or rooted cuttings.