Pain is super complicated. There are loads of different pathways and only some drugs can reduce pain in some pathways to some extent, either directly or indirectly. If you took a drug that could stop pain signals from reaching the brain, you also run the risk of stopping signals coming from the brain, or signals in vital organs. So, you could successfully stop all pain, but at the same time you'd stop breathing and your heart would stop beating. That sort of thing happens in opiate overdoses. As the user's brain gets accustomed to the weak signals to and from the brain, it overcompensates (pain gets stronger) as the drugs wear off, so the user is stuck in a cycle of pain relief until something vital fails.