Because they need to use very advanced differential geometry, such as Calabi's conjecture, which has just been proved for decades. The study of strings and membranes requires a deep understanding of differential geometry, which also gives birth to many interesting problems in geometry, such as the great development of complex geometry and singularity theory. In fact, string theory and M theory are not difficult in terms of their basic theoretical framework, because they both take the picture of modern quantum mechanics and treat classical objects as operators. Their difficulty lies in the imperfect mathematical description of these objects (strings and membranes). The most difficult thing is to improve the mathematical framework. Needless to say, many mathematical problems in the theory of relativity are very chaotic, such as the singularity of space-time (sp curvature singularity, geodesic incompleteness, etc. , are very chaotic problems, which may indicate the direction of quantum gravity), and some mathematical problems of quantum geometric dynamics (involving partial differential equations on infinite manifolds, which are difficult in mathematics). At present, it is not difficult to draw a basic physical diagram. The most difficult thing is to improve the mathematical foundation.