Why is that there were so many Cao generals who belonged to the imperial family but none of them gathered their troops to fight Sima Yi?
There weren't that many by this time (AD 249-250), thanks to some plotting by Sun Zi 孫資 and Liu Fang 劉放 in 239. We know from Sanguo Zhi, Wei-Jin Shiyu 《魏晉世語》(a 3rd/4th-century history book by Guo Ban 郭頒), and Han-Jin Chunqiu 《漢晉春秋》(a 4th-century history book by Xi Zuochi 習鑿齒) that other senior members of the Cao clan, such as Cao Xiu's son Cao Zhao 曹肇 and Cao Cao's son Cao Yu 曹宇, were originally meant to share the regency with Cao Shuang. Sima Yi was not even included in this original selection of regents. But Cao Rui changed his mind at the last minute, and appointed only Cao Shuang and Sima Yi as regents. Why?
When Cao Rui was dying from his illness, he originally intended to appoint Cao Zhao, Cao Yu, Cao Shuang, Xiahou Xian 夏侯獻, and Cao Cao's adopted son Qin Lang 秦朗 as joint regents for Cao Fang. But Liu Fang and Sun Zi, two powerful ministers in the Palace Secretariat 中書省, were on bad terms with Qin Lang, Xiaohou Xian, and Cao Zhao, and accused them of flirting with Cao Rui's concubines during his illness. They also warned Cao Rui that Cao Yu would seize the throne if made regent over the underaged Cao Fang. Cao Rui believed them and asked who should be regent then. Sun Zi and Liu Fang said Cao Shuang could be trusted, and also recommended Sima Yi to be joint regent with Cao Shuang. Some scholars have inferred from this that Sun Zi and Liu Fang were supporters of Sima Yi even before this
Cao Rui followed the advice of Sun Zi and Liu Fang, and ordered Cao Yu, Cao Zhao, Xiahou Xian, and Qin Lang to retire from office. From then on, Cao Shuang and his brothers were the only members of the Cao clan to hold political or military power (however, Xiahou Xuan and Xiahou Ba were close kin of the Cao clan, and were members of Cao Shuang's political faction). The self-interested actions of Sun Zi and Liu Fang were responsible for the rivalry between the factions of Cao Shuang and Sima Yi, but Sun and Liu themselves stayed out of the conflict by retiring from office in the late 240s. Liu Fang died in 250, and Sun Zi died in 251.
Shortly after the 249 coup, Grand Marshal Wang Ling 王淩 (nephew of the Wang Yun who plotted Dong Zhuo's assassination), who was then commanding the Wei armies south of the Huai River, began plotting a rebellion to replace Cao Fang with Cao Cao's son Cao Biao 曹彪 as emperor. Member of this plot included Wang Ling's nephew Linghu Yu 令狐愚, the governor of Yanzhou 兗州; Linghu Yu's general Zhang Shi 張式; and Wang Ling's general Yang Hong 楊弘. However, Linghu Yu died in late 249 and was replaced as governor by Huang Hua 黃華, and when in 251 Wang Ling sent Yang Hong to try and recruit Huang Hua into the plot, Yang instead betrayed Wang and joined with Huang Hua in reporting the plot to Sima Yi. Sima Yi marched against Wang Ling with the full imperial guard from Luoyang, and Wang Ling knowing he was outnumbered decided to surrender. He later committed suicide with poison when being sent to Luoyang as a prisoner. Cao Biao, who had been in contact with Wang Ling and Linghu Yu through Zhang Shi, was ordered to commit suicide one month later.
In 254, after Sima Yi's death, Palace Secretary Li Feng 李豐 plotted with the empress' father Zhang Ji 張緝 to overthrow Sima Shi and replace him with Xiahou Xuan. The plot was discovered, and Li Feng, Zhang Ji, and Xiahou Xuan were all executed. Sima Shi also replaced Cao Fang with Cao Mao shortly after this. No member of the Cao or Xiahou clans was involved in any subsequent rebellions against the Sima faction, except for Cao Mao himself.
Cao Zhao died sometime in the 240s, before Sima Yi's coup. Cao Yu lived on into the early 260s, but was never involved in politics after his retirement in 239. He probably knew that this was the only way for he and his sons to live to a ripe old age. But his son Cao Huan did become the last Wei emperor in 259, after Cao Mao was killed trying to overthrow Sima Zhao.