Because The Flash is a relic of the now broken Arrow universe. From “Arrow”, series such as “The Flash”, “Legends of Tomorrow” and “Supergirl” were derived. “Arrow” itself persisted for 8 seasons and 170 episodes, and the final season was cut to end after the end of the linked drama “Crisis on Infinite Earths”.
And “The Flash” has just finished its eighth season, and has surpassed “Arrow” to become the longest-lived work in the Green Arrow universe.
Moreover, considering that the production time of the series has been extended due to the impact of the epidemic, the actual length of time spanned by these eight seasons is even longer.
In any case, the eighth season of “The Flash” has ended, the total number of episodes has reached 171, and the ninth season has been determined to be produced. It seems that the show will continue to go on.