Why? Would you say the same thing about, say, ergodic theory, languages, automata and computation theory, analytic number theory, harmonic analysis, bordism and cobordism, algebraic K-theory, de Rham cohomology, Diophantine approximation, model theory and theory of forcing, descriptive set theory, algebraic geometry, representation theory... I could go on like this forever.
People tend not to know that most of the stuff a math graduate learned was already known by 30s, and category theory was only starting to become anything close to popular in 50s - 60s. People also tend not to know just how much math research have been done, especially in last 70 years.