Answer may be:

Flexibility is not a precondition to do yoga.

Over time, yoga can help your body to become more flexible. If you are not comfortable in doing some yoga asanas, they can be easily be modified based on the level of practice. So not being flexible should not stop you from practicing yoga asanas. Yoga poses work on increasing the elasticity of your tissues well as a stretch reflex and other involuntary nervous system functions. By continuous practice and focus on breathing one can get desired results. This continuous stretching loosens up the body by lengthening the muscles and lubrication of the joints allowing you to find more stability and flexibility, and gradually increasing your stretching ability. Just as lifting weights build muscles over time similarly consistent yoga practice increases the overall flexibility of your body. For a beginner start by practicing 3 days a week and gradually you can go up to 5 days a week. It all depends on the time you spend on practice.