It would be incredibly difficult to armor a launch pad to survive an explosion on-pad, so they aren't. The real solution is to have multiple launch pads (the space shuttle had three), so that if one blows up on the pad you have backups you can use until the blown up one is rebuilt.

The energy released in the first few seconds of a controlled launch is not remotely comparable to the energy released by an entire rocket blowing up simultaneously. Also, with an explosion, the entire rocket, along with parts of the strongback and other structures it's attached to, become shrapnel. Superheated water exhaust is a lot easier to protect against.