Why no library files installed for google test?

Is it a bug?

No, it's deliberate:

gtest (1.6.0-1ubuntu2) precise; urgency=low

  * Stop distributing static library (although still build it, to ensure gtest
    works).  Upstream recommends against shipping the libary at all, just the
    source. (See: http://code.google.com/p/googletest/wiki/FAQ)
    The Debian maintainer plans to do this also (see BTS: 639795); do it in
    Ubuntu now to fulfil MIR requirements.

To build static libraries

cd /usr/src/gtest
sudo cmake .
sudo make
sudo mv libg* /usr/lib/

---

Edit:

The names have changed slightly over the years, though the process remains the same. In Ubuntu 17.04:

sudo apt-get install libgtest-dev
cd /usr/src/googletest/googletest
sudo mkdir build
cd build
sudo cmake ..
sudo make
sudo cp libgtest* /usr/lib/
cd ..
sudo rm -rf build

Improving on izx's answer I would have used cmake this way:

sudo cmake -DCMAKE_BUILD_TYPE=RELEASE .

and I would attempt an out-of-source build:

cd /tmp
mkdir .build
cd .build
cmake -DCMAKE_BUILD_TYPE=RELEASE /usr/src/gtest/
make
sudo mv libg* /usr/lib/