Indeed! But using your figures, if I was the landowner, having to mind someone else's sheep all winter and feeding hay when it snowed, for a maximum of £12.50 a week, or have horses, I'd rather have three or four horses for a minimum £10 per week per head with no requirement to look after them and have to top, harrow and even possibly re-sow in patches as I went along. The price difference is only justifiable if you do that.
I'd even go as far as try to educate the horse owners so they could help....
As I say, people try to put too many horses on land and then don't help the owners to keep the land in good order. Apart from ignorance, not many horse owners have the equipment to care for land so have to rely on contractors who mostly spurn small acerages and deride horse owners.