Short answer: Because there are several ways of linking, and findlib is not the right instance to find out the right way
Recent versions of OCaml support DLLs, at least for some platforms. Here, the option -custom is not necessary at all, because the C libraries can be looked up and loaded at runtime. The option -custom would have the effect of forcing static linking.
But even for platforms without DLL support, there are two alternatives. One possibility is to use -custom, and the other is to create runtime systems with -make-runtime, and reference them with -use-runtime. Fortunately, recent versions of OCaml select now themselves -custom automatically if -make-runtime is omitted, so findlib needs not to bother with it.