Get ready to wave those hands
How active do these giants need to be? I ask because Wikipedia's list of tallest people notes a lot of health problems. The 25 tallest people who are no longer living had an average lifespan of only 39 years. Below is a photograph of John Rogan, the second tallest man in history. He's sitting because he was unable to walk after the age of 14.
Joints, bones, and muscle take a real pounding in extremely tall people, doubly so if your story involves extensive physical activity. It's possible to evolve a structure to support that kind of weight (lots of mammals do it) but it would take some re-engineering, either through many generations of evolution or an extra dose of handwavium. You'd also have to have a heart and lungs powerful enough to supply the farthest reaches of a 15-foot body with oxygenated blood. That requires more handwavium. Shorter people also have lower rates of afib (source) and, as listed by a 2013 study, other heart problems:
When shorter people are compared to taller people, a number of
biological mechanisms evolve favoring shorter people, including
reduced telomere shortening, lower atrial fibrillation, higher heart
pumping efficiency, lower DNA damage, lower risk of blood clots, lower
left ventricular hypertrophy and superior blood parameters.
The conclusion is that you could theoretically have super tall people, but they might be riddled with health problems and have trouble being very active.