Nobody is giving anything, the state is LEASING the rights to build, operate, and maintain the road. A private company is paying more than $7 billion for that lease, but the state will still own the road.
Not really any different than if a city leases out its garbage collection or when the state let's private companies bid on road construction projects. In this case instead of taxes paying for the construction, the private company will front all the construction costs and then recoup that and a profit through tolls. BTW, the 50-year lease can be canceled by the state at any time.
Secondly, it is going to lower the wages of American truckers because Mexicans will be driving the trucks.
Thirdly, no real American wants to be merged with Mexico and Canada with a super highway.
Fourthly, once they make these corrupt arrangements, like a fifty year lease, they never get rid of them.