Because unless they submit to the righteousness of Christ by faith, it is all in vain. All our good works must spring a from a pure heart, sanctified by faith in Christ's work on the cross. He despises every book, every sermon, every doctrine that calls men to holiness through man-made rules and traditions. Entire holiness movements have been born through men's desire to do everything humanly possible to please God and be holy. A few have become absolute legalists.
Church history is filled with ridiculous ideas on how to be holy.
I have read of monks who slept on bundles of thorns and piles of broken glass. Others bound one foot, hopping around on one foot until they lost the use of the other. Simon Stylites stood for thirty years on top of a column, and when too weak to stay there, he had a post erected and chained himself to it. In the Middle Ages, long processions of flagellants traveled from country to country, moaning, weeping, singing sad songs of repentance, and whipping their bare backs as they marched. We're just as guilty.