Perhaps big pharma has a strong hunch that a lot of small biotechs are based on a single good idea that could easily prove worthless in the long run, or in the short term.
Or, even if it is a really good idea, it takes a huge amount of money to put it through Phase II and III stage clinical trials, so big pharma can afford to sit back and wait while small companies go through growing pains before they decide to cherry pick potential winners.
Keep in mind, being a CEO or CSO or CMO at a small company pays far more than any salary a junior professor makes, which makes starting your own company quite attractive if you can swing it.
In addition, if you get bought by big pharma, you may be shown the door, so, even with a golden parachute, it’s not the same as being in charge.