@a1ba >because it's considered as a system library and goes into an exception.
The LGPLv2.1 does not have a the "system library" exception, it has a different one;
`For an executable, the required form of the "work that uses the Library" must include any data and utility programs needed for reproducing the executable from it. However, as a special exception, the materials to be distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.

It may happen that this requirement contradicts the license restrictions of other proprietary libraries that do not normally accompany the operating system. Such a contradiction means you cannot use both them and the Library together in an executable that you distribute.`

Such exception does not give unlimited permission to link to glibc - it merely allows compiling and linking glibc on a proprietary system.

https://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html

>It's good that glibc is GPL licensed
There is no such thing as "GPL licensed", there is the GPLv{1,2,3}, LGPLv{2,2.1,3} & AGPLv{1,2,3}, all of which differ in detail.

>I wish GPL actually covered non-obfuscation case better because GNU people seem doesn't know how to write a readable code.
The GPLv3 makes it clear that obfuscated code is object code and not source code; `The "source code" for a work means the preferred form of the work for making modifications to it. "Object code" means any non-source form of a work.`

>GNU people seem doesn't know how to write a readable code.
When I read GNU software, all I see is pure freedom, which is plenty readable.