Planting projects on a small scale can easily be done by hand. Once projects on a bigger scale are done, this becomes difficult. As an example we take an imaginary project. The client wants to plant 600 weeks x 1 million trees (2 million hectares x 300 trees / 12 years)

That is 1 million planting holes per week.

Suppose each man makes 4 planting holes per hour, working in a high temperature, with rocks in the soil, than you need 250,000 man hours per week. That is 6,000 man to dig planting holes. Plus the transport (120 buses), housing, food, organization and management. This is not easy to organize.

Info: in Morocco the government does some planting projects 500 km south of Oujda, deep in the desert. They calculate 2 planting holes per man per day. Maybe that is very low, but it is their experience. With 6,000 man and +45°C, 4 holes per hour is a definitely a challenge.

The cost price of tree planting holes made by hand
Suppose a man's wage is 3 US$ per hour. The cost of making a planting hole by hand if made 6 per hour - excluding the transport (120 buses), housing, food, organization and management - is 0,50 US$.