Because at one point one recruiter or HR manager will ask him to provide a reference from his previous manager? And when he looks like an absolute ass he can pad his resume with whatever he wants? Short time thinking will not get rewards in the long run in that case.
The reason we have this phenomenon is that companies highly value (or even overvalue) experience with hot new technologies. So even if as a developer you know that maybe kubernetes, microservices, and NoSQL [or whatever it is] are not right for your organization, you know that having those on your resume will help you get a big raise and a promotion on the next job. The chances that your new tech chasing misadventures will haunt you in future jobs is very low.