Not denying that acetaminophen/paracetamol has a horrible therapeutic index, but there are reasons why it is still so widely used. Despite potential hepatoxicity, it is considered the safest pain relief drug (this emerging research notwithstanding)
Ibuprofen is not better in every way. For instance ibuprofen, as well as other NSAIDs, is much less gentle on gastrointestinal tract and puts much more strain on kidneys.
Some people (e.g. with inflammatory bowel diseases or renal diseases) do not tolerate NSAIDs well, which is why acetaminophen is on the same step of analgesic ladder - it is still considered better in some cases. Both are also used complementary to each other in pediatrics.