This "vanishingly small" number is still way, way too high. Five in a week? If there's one thing you don't want your electronics to do it's potentially kill people.
There was a rash of exploding battery cells in the 2006-2007 era where Apple even issued a recall. At the time Apple was still using off-the-shelf cells and hadn't started to make their own batteries. This affected many other vendors using the same suppliers.
Even Boeing's planes were not immune to battery problems, with the 787 famously suffering some serious issues, including fires.
Battery technology is something that's very hard to get absolutely right, to render something safe enough it won't explode. You want a lot of "sigmas" in your quality to avoid any serious defects.
I don't know what Apple does to their batteries, but the sorts of abuse they can apparently endure without catching fire is remarkable: https://www.youtube.com/watch?v=iy5kAY5SqEk