Why did Al-Bukhari and Muslim not mention the hadiths that weaken good deeds, which are found in the Sunan books and Sahih Ahmad, such as the hadith about ghusl on Friday?

Al-Bukhari and Muslim did not include all sound hadiths in their books, and they explicitly stated this. We narrated from Al-Bukhari his saying: "I did not include in my جامع (Jami') book anything but what is sound, and I omitted from the sound ones due to the tedium of length." And we narrated from Muslim his saying: "Not everything that is sound to me have I placed here; rather, I have placed here what they (the scholars) have agreed upon." They did not commit to including all sound hadiths; rather, their intention was to compile a selection of sound hadiths. If they omitted a seemingly sound hadith, it is likely that they discovered a defect in it, or they omitted it due to forgetfulness, or in preference for avoiding lengthy exposition, or they saw that another hadith could serve its purpose. Therefore, there is no issue with their not including many sound hadiths concerning the virtues of deeds and other matters.