What is Yoga? If you ask 10 people, you are likely to get 10 different answers. If you ask someone again after another 10 years, the answer is likely to change but will still vary significantly from person to person. Naturally, one's opinion is informed by the studio one goes to or the person one learns from. If you try to learn Yoga today, at most studios you will be told that what they do is Yoga and you should just keep coming and results will eventually happen. Rarely will you be educated about different styles and which one may best suit you, and almost never will a studio talk about how their approach differes from traditional Yoga. This isn't done out of malice, it is done because most teachers today haven't received a training that would allow them to answer these questions.
Today's teacher trainings focus on giving aspiring teachers the basic ingredients that allows them to lead a class that people like. Studios hire teachers who teach popular classes so the business is successful. The more people in the room, the better for the bottom line. So teacher trainings focus on poses that are challenging but not too challenging, sequences that flow nicely (generally called Vinyasa) and are choreographed with motivating music, and a bit of philosophy so nobody confuses the activity with more mundane gymnastics. Teachers may spend years refining and changing their sequences and playlists but more often than not remain unaware of how different their approach is from traditional Hatha Yoga.
If you ask a Yoga teacher what Yoga is about, you are likely to get answers about stretching and strengthening, building one's core, etc. So how does this differ from gymnastics? If a gymnast trains from early childhood on, one can assume they achieve a level of core strength and flexibility unachievable by anybody walking into Yoga studio as an adult. But if Yoga is about stretching and strengthening, clearly gymnasts should be great Yogis. Yet physical pain is quite common among career gymnasts as they get older. And a gymnast may laugh at the idea that these physical exercises are some spiritual endeavor. Somehow gymnastics is considered physical but Yoga is spiritual? Repeating "downward dog" or "triangle pose" thousands of times is supposed to bring you closer to enlightenment? How would that even work? Clearly, something is missing here.