They simply couldn't do it.

Even gaining control of the whole mainland and organizing their forces to attack Formosa, the PRC simply did not have the naval or air forces required. They didn't have enough military landing craft in the navy and there were not enough local fishing boats in either number or quality to make the journey. Finally, the United States Seventh Fleet was stationed in the straight to prevent the PRC from attacking Formosa, especially during the Korean War. The US didn't want the Chinese to take advantage of the situation during the summer of 1950.

China's entry into the Korean War was to make it a Chinese-American war. While North Korea and the USSR assumed that the US would not intervene in Korea, when they did, China (PRC) wanted to use it as an opportunity to fight the US. The US had/was backing their enemies in Formosa and they wanted an opportunity to get the US to get out of the mainland so that they COULD retake Formosa in the future. They underestimated anti-communist sentiment and the "China hands" in Washington and it didn't work - the US held the line in Korea, they maintained a blockade, protecting Formosa and the ROC, and they continued to recognize the ROC as the one true government of China until the early 70's when the One China policy came into effect.