The professor probably had other options, but decided that this is the route to go. And once the decision is done, there is no turning back. In the early phase

The professor probably had other options, but decided that this is the route to go. And once the decision is done, there is no turning back. In the early phase of a startup it quite often happens that equity is given on very relaxed way on good terms. For example someone can work only short time in the company and still get nice piece of equity, just because other founders are lazy or careless. A lot related to startups is "play stupid games, win stupid prizes". Sometimes you can get lot of equity & money if you just happen to be in the right place at the right time. Other times you end up working a lot and get nothing.