We have a fairly firm grasp on exactly why. The universe is finitely old (rather, has looked basically the way it does for a finite length of time) and rapidly expanding. Either could handle the issue alone, but both make sure of it. We are independently confident both in the fact that the early universe looked like a hot, roughly uniform soup and that the universe is currently expanding. The whys and hows are less clear on both fronts, but these at least are physical observations we make. My astrophysics friends inform me that there's lots of debate on the exact timeline and if inflation happened the way that it is often described, but my physics knowledge tends smaller so that's about the limits of my knowledge, at least until someone finds a way to slap a nice God-fearing operator over this whole mess.