In the US, it is hard to recruit postdocs unless you are at a top 10 school (I tried several times). Potential postdocs are largely incentivized by: (1) name recognition of their mentor since their letter and networking will get them their next job and (2) the likelihood of getting top-tier publications. Also, they get a decent salary and health insurance in engineering fields.

A brand new research institute does none of these things for them. I don't see why a fresh PhD would choose this over either FAANG or a university.