Because a wider, better resourced, and long term investigation would be better equipped to collect and analyze evidence? Because a better structured and mandated team would likely have more access, credibility, and ability to undertake that assignment? Because, as the report discusses, it often takes years or decades for crimes committed during armed conflict to come to their conclusion, for myriad of reasons?
Among other statements, here's what the actual UN report actually said about just this: "As in other conflict-affected contexts, there
remains a significant likelihood that the findings of the mission team, in terms of verified violations, only partially reflect the crimes actually committed. A more comprehensive assessment of the occurrence of conflict-related sexual violence in the context of the 7 October attacks would require a fully-fledged investigation by competent bodies with adequate time and capacity." (Page 15, section C, subsection 56).
Over and over again this report says that "there are reasonable grounds to believe that sexual violence occurred" on that day, in various settings. I'm not sure why you think that this amounts to "rape definitely did not happen."
And, since your counterargument rests on the idea that Pramilla Patten is just "a woman," I think you should think about who and what she is: a legal expert, practicing lawyer, and judge who has been investigating gender-based violence for more than 20 years, and specifically sexual violence in conflict settings since 2017.