Yes there is a clear reason why Lord Buddha himself didn't to that and forbid others to do it too.
As you know Prince "Siddhartha" (the prince who became our Lord Buddha) gave up all hopes of life and started his journey towards understanding. Let me tell you about how extreme that will was, Before giving up all food Lord Buddha ate only one "Mung" seed per day.
The reason why this rule was given has two sides....
If monks try to grow their own food and consume two questions arise. They will have to waste valuable hours to do the whole farming thing that they could use to further the path if there was no growing.
The other reason is they will obviously be drawn to grow what they like to eat and that will not help their path (Because of it feeding the attachment to taste).
And every time a Monk consume temple grown food that would be a chance lost to a lay person to gain some good karma.
Historical updating of the rule....
Long after the Parinibbana of Lord Buddha kings took things a bit seriously. They saw that in case of a national emergency monks are at risk. Because if people have no food they can't give to monks.
So kings came up with a good solution. They granted temples and monasteries with land & people to work. So the villagers would build houses and farm and in return share the harvest with the temple. As kings have already given the rights to the land and all upon it the rule does not break.
Modern day updating of the rule....
Now as the society is getting busier there is a good chance that a family might miss their turn to offer food to the temple. So there is a trend to innovate. No matter if they are giving their offering or not people now bring some supplies with them to the temple as offerings, things like Rice,vegetables,Soya,Dried fish and etc. As the offering have been made it is OK to consume then when the need arise.
Monks can garden if the Land and all upon it has been offered to the temple. It happens all over Asia. But monks keep themselves from digging and such tasks because of the possibility of harming an animal. Most of the times all the work is outsourced to laypeople and a share of harvest is given.