Who decided the hard lump of yogurt from the store was the appropriate texture, anyway?! I love the soft, silky, creamy texture of high quality raw milk yogurt. Here are some not-so-great reasons the "other stuff" is hard:
1) Powdered milk or whey protein powder is added in to get the milk solids high enough. Mass market milk is watery because of the types of cows used and what the cows eat. To achieve the relatively high percent of milk solids required to make good yogurt, they mix in powdered milk and/or whey protein powder rather than use high quality cows on good feed.
2) The milk is disassembled into component parts, then re-assembled accordingly to get the right ratio of protein. Protein is critical for gel formation, so milk for mass market yogurt has standardized protein levels. Rather than use good, healthy cows on high quality feed, this is achieved either with the addition of protein powders or disassembling and reassembling the milk components.
3) Stabilizers and thickeners are added to the milk to keep the yogurt from separating and to make it thick. This hides the clues as to the real quality of the milk. Also. some stabilizers, like carrageenan and gums, appear to be unhealthy for many people.
4) Milk for mass market yogurt is homogenized (which causes the fat to be unhealthy). Conveniently for commercial yogurt makers, the reduced particle size of homogenized fat causes an increased surface area for proteins to coat. This equates to more structure-building components for the gel matrix.
5) Milk for mass market yogurt is pasteurized. Conveniently for commercial yogurt makers, pasteurization denatures the proteins which enables more cross-linking of them and thus a stiffer gel. Also, yogurt cultures are negatively affected by oxygen and pasteurization removes the dissolved oxygen from the milk.
6) Mass market yogurt almost always has added sweeteners, and they're not natural or healthy. A side "benefit" for commercial yogurt makers is that this increases the total milk solids which increases the gel network and makes a firmer yogurt.
I never did like the hardness of commercial yogurt. Now I know why!