The answer lies in the soil

Droving took place before the age of nitrates or artificial feed.  In the hills of Wales & Scotland the grass was too poor to fatten beasts for the table – and ‘incapable of much improvement', as Adam Smith put it.  ( Even today, in much of the Scottish Highlands ten acres will only support one sheep.)


Therefore, most Welsh & Scottish tenant farmers could not fatten their own meat - let alone afford to eat it - so they were forced to sell their beasts in faraway markets to pay the rent.  They thus provided the ‘stores' (meaning cattle ready for fattening but not for the table) to be grazed on English pasture before being sold to butchers in London and the South.


Graziers in the English Midlands and Home Counties just wanted the 'frames' of the healthy beasts¹ .  Their job would be to put on the meat & fat, a job that might take anything from 3-9 months.


As the cattle and sheep swapped the acidic 'desert grass' of Wales and Scotland for the lush valleys of the English Midlands, they must have thought they'd arrived in Heaven. 

¹ As Twm Elias puts it in his On the trail of the Welsh Drovers.