Well, that's kinda the point. Many songs people are familiar with originated from other artists. There is a fine line between copying unaccredited and being inspired to create a version that truly is a different song based on an original. In this particular case, Memphis Minnie is credited on the song on Led Zeppelin IV, although many parts and lyrics are different. Also, many other artists continued to be inspired by the song after Zep's version and produced yet more, different takes on it: >Whether clued up or not on that fifth name, a bunch of notable artists went on to cover Led Zeppelin’s “When the Levee Breaks,” with the effect of giving it the added mystique of an original. Louisiana bluesman John Campbell, for instance, growled his way through the song in 1993, having found obvious inspiration in Page’s distinctive bottleneck-slide action. Alt-rocker Kristin Hersh similarly adopted the hypnotic groove invented by Led Zeppelin for her brooding acoustic rendition of 1994. Jeff Buckley, meanwhile, captured the whole Zeppelin vibe on a live performance of the song from his peak 1995 period, notably by matching Plant’s acrobatic vocals, and letting rip on the slide guitar. Thereafter, glam-metal band Great White replicated Page’s arrangement of the track on their Great Zeppelin tribute album of 1998, and even supergroup A Perfect Circle based their radical and trippy reinterpretation of 2004 on that familiar doomy mantra of 1971. I agree Page had several instances where he was rather reluctant to readily credit a song he was "inspired" by, but he/Zep are hardly the only ones to do so. There are many examples in Rock history.