Can someone explain why Twitter's tech isn't better able to spot fakes which seem to follow obvious patterns? ProPublica was able to write a program to analyze

Can someone explain why Twitter's tech isn't better able to spot fakes which seem to follow obvious patterns? ProPublica was able to write a program to analyze the networks and patterns of behavior: We found a pattern of coordinated activity among the fake accounts that appeared to be aimed at building momentum for particular storylines. Central accounts with more legitimate-looking histories such as Keegan’s would make eye-catching posts; for example, a political message accompanied by a bold graphic or a meme, or a provocative video. An army of obvious fake accounts would then engage the posts with likes, reposts and positive comments, presumably to boost their visibility in Twitter’s algorithms. Why does it take an outside party to build the tech & point out the problem? Twitter's response, FWIW: “Using technology and human review in concert, we proactively monitor Twitter to identify attempts at platform manipulation and mitigate them."