The issue is that you might not end up at the desired ecosystem you want. While natives are good, you need to consider topographic, climatic, and edaphic conditions to be successful. It doesn't make sense, for instance to plant a bunch of cottonwood trees on sandy sites, because they'll die off. More importantly, however, it doesn't make sense to plant monocultures of native species as you don't end up where you want to. This second point is often what's done with commitments like this because it's easy, and planting diverse communities is difficult both in terms of physical labour, but also logistics. Some species need their seeds to be heat or cold scarified and that also takes time when you're wanting an assload of seedlings right away.

Even native trees can need help to establish. A common thing that happens with massive planting is the land chosen isn't really able to grow trees without additional water or nutrients, so within a year or two 90% of the trees are dead.

https://www.kcl.ac.uk/news/world-cant-rely-on-tree-planting-to-limit-climate-change-says-report