"Why" I can't say. But looks like with the 20.4 release you'll be able to show or hide completed work
Highly recommend turning on the Projected Dates (the enhancement Heather mentioned) as that will show if something ended early or if something will most likely be delayed because the rest of the project is running late. My Creative and Dev resource managers love that since they don't usually have instead in how the project is doing until it gets to them. So totally helps with prioritizing work.