Because if you don't agree to the GPL's interpretation of it, then you haven't agreed to the license for the library and you don't have permission to distribute it?
(edited)
I think this is the magic legal judo of the (L)GPL that makes it work. Regardless of your feelings about the license, it's extraordinarily clever!
Note that if the license said you had to, for instance, share x% of your revenue with the licensor, that's not part of copyright law either, but it's still probably a legal license.
Neither the GPLv2 nor the GPLv3 define what constitutes a derivative work or make any claims about linking in the normative parts of the license.
Let's assume that tomorrow some courts were to find that two separate works can link to each other without being considered derived works of one another, and without the whole being considered a derived work of either part. Nothing in the text of the GPLv2/3 would then prevent you from writing a program which links to GCC, and distributing it under a fully proprietary license (as long as you distribute the sources of the version of GCC you're using, of course).
There is one tiny reference to linking in the non-normative parts of the GPL, the "How To Apply These Terms To Your New Programs" section after the explicit "END OF TERMS AND CONDITIONS":
> If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library [emp. mine]. If this is what you want to do, use the GNU Lesser General Public License instead of this License.
This may suggest that the writers of the GPL believe this would not be allowed under copyright law, but being outside the normative area of the license, seems unlikely to be an impediment in court.
(edited)
I think this is the magic legal judo of the (L)GPL that makes it work. Regardless of your feelings about the license, it's extraordinarily clever!
Note that if the license said you had to, for instance, share x% of your revenue with the licensor, that's not part of copyright law either, but it's still probably a legal license.