Most of the complexity of a good malloc (jemalloc/tcmalloc) is going to be in its thread-local caches and in its placement heuristics, both of which are also necessary for a good GC (for the tenured generation, in the latter case). Accounting necessary for the tenured generation is also comparable to accounting for a malloc implementation. Fragmentation also isn't much of an issue anymore with modern allocation schemes. When you add in the complexity and overhead of tri-color marking and write/read barriers, the overhead often isn't in GC's favor, compared to a well-tuned program that uses the stack where possible and a good malloc implementation for long-lived objects only.