* they don't think the solution is overcomplicated
* they don't really care about the startup succeeding
* they don't know what the ramifications of picking the wrong technology will have
* they think the risk of learning something new is outweighed by the value to the company (or themselves)
I don't think startup employees are altruistic, but picking a tech you know will have a detrimental effect on your employer strikes me as shortsighted.
If given a choice between Vue and React, I might prefer Vue. But I would still choose React as a tech lead because I know it is more marketable and you would get more self interested developers and it would be easier to recruit for.
Technical correctness is not the only factor, there are other kinds of correctness, as you point out with React: "you would get more self interested developers and it would be easier to recruit for".