If an ambitious grad wanted to make their mark, they'd either apply to one of Google's institutions (DeepMind, Brain) or be generated by TPUs (if they were literally a grad[ient]).
Nowadays, not so much. Researchers are realizing that Google is the place to go if you don't want your research to be incorporated into products, except indirectly (Youtube recommendations, google ad placement, etc). Meanwhile OpenAI is reshaping the world.
It's in OpenAI's interest to strike while the iron is hot, as they say. This hype will die down eventually, but displacing 30 million-dollar researchers from Google will have long-term effects that are hard to measure.
Plus, PhDs are the resource that can't be replaced. If someone magically vanished OpenAI's PhDs tomorrow, it would be very hard for them to recover. Whereas if someone vanished every remaining Twitter engineer, Musk could probably pull off a recovery by hiring replacements.