Because if the Druid charged the going rate for casting a spell of that level the increased crop yield still would not come close to paying for it, let alone be profitable. Same can be said for a wall of stone, stone shape, etc...
So most of these spells would be cast for the personal benefit of the caster. IE the Druid is a farmer, or buillding a stone structure for himself (stone shape).
Where I really see spells messing up economics is when they can summon elementals or xorn and command them to find the nearest deposit of gold, platinum, silver, mithral, adamantium, iron, corundum, diamond, spinel, beryl, etc.... Granted it may take a few castings, but eventually they will find it. Then a few move Earth's, rock to mud, maybe a passwall or two, and EUREKA!
Of course I ran a very intense game all centered around finding a rich gold mine, because of an aurumvorax. Then a very intense game of fighting the Zhentarim and other power groups, such as the Kraken Society, realizing they will be overwhelmed sooner or later, and being smart enough to bring in "partners" to help establish a "balance of power".
The group still ended up rich, but only millionaire rich instead of billionaire rich. But it ended up funding their entry into the epic levels nicely.