A government owned company has easier access to public land and can do that while doing their public infrastructure work. It can go as a more coordinated work and costs can be saved greatly, especially when the property is publicly owned. The work itself is then normally done by private companies in a bidding process.
The government own companies faces only problems, when it has to cross private property.
While a private company has to pay for the usage of the public property too. It has also to work on the allowance to use the public property at all. Also the construction costs are much higher, as they cannot share with other infrastructure costs. If private companies wants to share the costs of infrastructure work, it requires much more coordinating work. They also need to know way in advance when which infrastructure work by which private company is planned to be able to work together. That will become a bureaucratic mess.