People decide for themselves. During patient recruitment there's an informative phase.

The patients who are interested in participating, and meet the eligibility criteria, have to be informed about the risks. That's meant to fulfill the informed consent requirements.

Many candidates choose not to move forward based on the risks. Or if they did enrolled, if other patients have poor outcomes or adverse incidents, or if they personally don't see improvements they can drop out.

Also, some patients just die, or have secondary health incidents that force them to cease their participation.