Yoga asanas incorporate stretches, but also balancing and strength-building exercises. Different styles emphasize some features more than others. Yin Yoga, for example, concentrates on deep stretches, which are supposed to activate ligaments, whereas Hatha strives to have muscle stretches, balance, and strengths. There are various traditional yoga practices as well as modern fusions (aqua yoga, aerial yoga).
Nowadays yoga is mostly viewed as a type of physical exercise, to lose weight or to build insane flexibility which could be showcased on Instagram and collect many likes and followers. Once yoga teachings became available to the west, people started exploiting it to improve their self-image and gain popularity or stand out from the crowd (look at what I can do!).
Unfortunately, it seems that the western world trashed most of the traditions that yoga carries instead of respecting access to the information which was carefully preserved for over 5,000 years!