Which is exactly why taking short term unemployed developers is difficult to execute correctly. They will not be unemployed for long, which means that they get up and leave the project - taking with them domain knowledge. That knowledge is extremely expensive to pass on - it involves the startup having to "train" their next employee. Better to reduce churn and get permanent developers.
This model only works for simple greenfield projects; as soon as you get complexity, it introduces dependancy of domain knowledge, which doesnt work with short term employees.
Even in opensource projects, there are usually only a few core developers - i.e. the ones with domain knowledge. Everyone else submits small bug fixes/patches.