I think that the word you should focus on would be "importance" rather than "relevance," because I believe the issue behind your question is that people do not believe that mathematics is important in physics. As a matter of fact, my son (an engineering student) found that his physics courses in high school actually were the most helpful courses he had to prepare him for his college mathematics courses. Here are some other topic questions related to this one:
1. What is the importance of studying math to be prepared for learning physics?
2. How does mathematics research illuminate or drive physics research?
3. How can mathematicians and physics researchers work together to solve big problems?
4. What are the most interesting mathematical problems in physics research today?