The hope is that this will incentivize and result in more development of the American manufacturing sector

Tariffs alone are not enough. Expanding production is a long term investment, but there's no guarantee a tariff will still exist in 4 years. Its risky to invest capital to meet tariff-induced demand when that demand might disappear before your new factory is even finished being built.

Uncle Sam can mitigate that risk by subsidizing the construction of new factories while the tariffs are in place.