First, someone like me flagging this has killed or crippled potential acquisitions by big tech companies. In most cases, the startup has no idea that the plagiarism was discovered but it follows their reputation. It is a giant red flag since it raises questions about the provenance of the rest of the IP. Plagiarism reduces your probability of a successful exit.
Second, in some cases, it puts you in the position of tacitly trying to execute someone else's technical vision that you may be lack the expertise to replicate. I've also seen this failure mode multiple times. Plagiarized documentation makes a promise that will be difficult or impossible for the startup to deliver on because they lack specialized expertise that the company they copied it from has. This is a form of self-sabotage since it puts the startup in the position of executing from a position of weakness in order to match the content they ripped off from some other startup, which leads to poor product and poor customer experience.
Copying abstract ideas is fine, there are many possible implementation variations, but blatantly ripping off other startups is a very low ROI strategy for startup success.