No one ever said live rock is the only filtration system you need on a tank. It helps, but it shouldn't be depended upon to do everything. There are many aspects and solutions to filtration including equipment (skimmer, reactors, sumps, refugiums, filter media, etc.), livestock (amount of fish, type of fish, use of snails/hermits, etc.), and your personal maintenance habits (feeding, water changes, quality of water used, testing, etc.). Live rock is great, but again, I don't think anyone ever said it was all you need to have a successful tank. No two tanks are exactly alike so it is a balancing act you must figure out. What works for one person, may not work for another, and vice versa.