This is very common problem , the root cause is the spark is jumping too large a gap as an rotor comes around to meet the post inside the distributor . This happens due to a variety of problems most commonly someone has installed the distributor incorrectly and even though it's a two phase or two off the spark will jump a very large gap and that's what builds up the corrosion and eats away at the cap . If the distributor has not been removed or installed then this is a very good sign of a Worn distributor gear , Faulty distributor , Wrong cam sensor , Or just a miss adjusted distributor
Install a scanner and look at the cam retard value with the engine at 1200 RPM in park. The factory specification is 0 degrees plus or minus 2 degrees. If not in specification, turn the distributor slightly, then bring the engine over 1,200 RPM again and the value should update. Repeat this process until 0, plus or minus 2, is achieved. If the distributor has been replaced, verify the correct distributor has been installed. Remove the rotor and check for an "8" stamped on the cam position sensor shutter blade/rotor plate if equipped with 8 cylinder engine and "6" if equipped with a 6 cylinder engine.