Those exist and are in active use by physicists (in fact one of the first CAS called Schoonschip was developed by nobel price winner to be Veltman to carry out Feynman diagram calculations). However there remains a huge tome of high energy physics to be implemented in one common CAS (one of the things I sometimes dream about starting). The culture of theoretical physics is such that only the end results of calculations are ever shared with intermediate steps left as exercises to the reader. Graduate students are expected to develop a stash of identities and routines to go back to, which is their “competitive advantage”, especially in fields like model building.