There are several reason that projections are over 10 years. The primary reason is that many initiatives take several years to implement, so using a 10 year period can more accurately capture the effects. For example, building a four lane bridge takes years of planning and construction, and will likely cost billions. After it's built their are possible tolls and increases to economic activity that offset some or of that cost, three or 5 year projections may only capture the construction phase and no benefits. 10 years is a bit of a compromise between long enough to capture effects and short enough to still have some accuracy.
A more nefarious reason to use the 10 year standard is to hide the true costs of doing something. Many bills that claim to be "budget neutral" over ten years are front loaded with spending in the first few years, then have some combination of new taxes, economic increase, or fees balance out those first years of spending. Then in the later years the proposed tax is canceled or the income is reallocated to some other project, or the growth never happened so money was spent and the ROI never happens. Additionally by using a 10 year window a bill that is really 9 billion over two years in spending can be called a 10 billion over ten years by spending a nominal amount for the latter years with most being spent early on.