No one builds batteries expecting them to explode in normal use. But li-ion battery manufacturers know it's statistically probable that a small percentage of their cells are going to blow. They still sell these batteries because they've calculated that the failures will be few. This is done with something called the MTBF, or mean time between failure.

The way MTBF is measured has little to do with the way the batteries are used in the real world. It is a sham. A random sample of batteries, say 1,000, are put to use in a target device and operated until one or more fail in service for whatever reason. If it takes 100 hours for the first battery to fail, the MTBF is set at 100,000 hours - that is, 1,000 times 100, an essentially meaningless formula, but a satisfying number for all concerned, as it implies that if your cell phone were to explode it would most likely do so after thousands of hours of use.