Going one by one would present decision fear to any number of said engineers. "Am I going to be offered this deal as well, or will I be left out?" By going at

Going one by one would present decision fear to any number of said engineers. "Am I going to be offered this deal as well, or will I be left out?" By going at it collectively, it reduces perceives downside risk. Also, it reduces Square side's risk that one or more key people it wants would balk at the individual offer. By offering a group deal, it pressures everyone to join, since human nature is to not want to sabotage the communal good for your own benefit, especially when you've worked together for years. It may also permit a more holistic retention package for the next 4 years for this team and keep VCs happy.