how is uber getting all the best hires?

Uber is hiring A LOT of engineers.  Ask any recruiter or hiring manager in Silicon Valley and they will tell you they have lost at least a couple engineers to Uber in the last few years.

That said, they aren't necessarily hiring "the best."  As the "fastest growing tech company ever" during a time when talent is more difficult than ever to hire, Uber has to make big sacrifices. 
They have hired some great engineers, and they have also hired LOTS of mediocre engineers that wouldn't get hired at your average startup.