Why does gcc not implicitly supply the -fPIC flag when compiling static libraries on x86_64

1. See question 3544035. Also discussed here and there.
2. It depends on what use you will have for your static library. If you only want to link it into programs, it doesn't need PIC code (libtool calls that a convenience library, because you could pretty much do without it, it simply helps get your compilation process to a reasonable size, for example). Otherwise, if you intend to link shared libraries against it, you need PIC code in your static library.
3. See question 3146744 and also here
4. It bloats your code, so it's not the default. One thing to see is that, when you compile a single object file, GCC doesn't know if you're going to create a shared library out of it or not. In most of my smaller projects, I simply link together a couple of object files, and do not need PIC code, for example.

Also, my advice would be: if you need to worry about that, you're doing it wrong (or you like to learn the hard way, which is nice because you'll get more out of the experience). Compilation systems (libtool, cmake, whatever you use) should do that for you.