Either your opencv build did not install into python3, or you have a customized python3 installation and it has been installed in a different directory than the actually looked one.
I have poor experience with python setup and may not be able to help much further.
I can just share my own case:
# Get your python3 version
$ python3 --version
Python 3.6.9
# Find the lib that provides python bindings for opencv in your python 3.6 dist-packages.
# Note that depending on your setup you it may not be in /usr/lib/ but may also be in /usr/local/lib/
$find /usr/lib/python3.6/dist-packages/ -name 'cv2*.so'
/usr/lib/python3.6/dist-packages/cv2.old/python-3.6/cv2.cpython-36m-aarch64-linux-gnu.so