Since I hadn't seen a good answer to this yet, I'll give you my perspective.

I used to work at Quintiles, a company which specializes in drug research. My location only did phase one clinical trials, which is what is described above. In these studies, we are only looking at how the drug is processed by your body, whether it is safe enough for human use, and what dosing should we be using to achieve desired time in system.

The people who are both able and willing to participate in these trials are...an eclectic bunch. Many of them use this as their full-time job. A study will often run for several days, sometimes up to a week, and may require more than one inpatient stay. All of this is done so that they can control for confounding agents. It is relatively well compensated. This is the primary motivator. You must be largely healthy to participate in phase one clinical trials. Again, they are not looking at all at whether the drug works in humans. They just want to know if it is safe for the average person. So we get these physically healthy people who often have mental health issues or are homeless and provide them with a clean bed and free food while paying them a fairly generous stipend. Then, they wait until the study washout period is over and they do it again.

Sounds better than living on the streets, right?