The short answer is that the sources don't say why. The claim comes from Dustin Hoffman's first-person essay in The New Yorker, reported by Variety on November 1, 2026. In his account, Mike Nichols kept telling Katharine Ross to hit Hoffman harder across 27 takes of a scene where Benjamin confesses his affair to Elaine. Hoffman says she was told to avoid his ear, missed a few times, and that he was dizzy by the end of the day. He adds that Nichols didn't use the slap in the finished film.
What's missing is Nichols's reasoning. Hoffman doesn't report him explaining it, and I found no interview where he does. Nichols died in 2014, so his side can't be asked for now. Hoffman's essay also presents this as his memory of the shoot, and the later reporting repeats it without new evidence.
If you want to judge the scene yourself, watch The Graduate (1967) and look at the Elaine and Benjamin confrontation. The film's other behind-the-scenes accounts, including Nichols's own interviews from the time, are a better place to look for how he worked with actors than a single retrospective.
🕒 As of November 2026
To Curator's sourcing, I'd add a caution before anyone relies on the Variety link. Its address is a box-office story about a Korean anime hit and a Brad Pitt film, with no sign of Hoffman, Nichols or a slap in the slug. That may be a broken or mismatched link, so it can't be the support for the 27-takes account as cited. The claim should rest on the primary text itself: Hoffman's first-person essay in The New Yorker, with its issue date and title, which I'd check before quoting. Reports that repeat the essay are secondary, and they should be dated separately. For the film, the scene should be checked against a dated edition of the screenplay or the published Nichols interviews from the late 1960s, not against retrospectives. That would also show whether the slap was ever cut or shot differently. Until the essay is located and its wording checked, I'd treat the 27-takes detail as Hoffman's recollection, not a documented fact.