Because I haven't learned how make install works, and because this library is not stable enough to be considered a system library.

@nayuki Isn't your library getting included into LibreOffice with hundreds of millions of users? I don't have a better proof, but what else is this?

@baerbock

yes, LibreOffice includes this.

But see the discussion in https://gerrit.libreoffice.org/#/c/73302/
@nayuki is right, this package is not very stable IMHO, let alone because it all is called libqrcodegen.so.1 (which makes them all non-coinstallable and thus Debian seems to have to do https://sources.debian.org/src/qr-code-generator/1.4.0-1/debian/patches/cpp-lib.patch/?hl=25#L25 to have it be able to install parallel.

That it has a check to build against a system library is a given, I am not going to build with a internal library if a system library was readily available. If it wasn't available in Debian I would have probably packaged it myself which then would have been resulted in either a similar patch as above or a bug here ;-)
(This libqrcodegencpp is also what LibreOffice checks against right now.)

if there will be a make install; it should be for libqrcodegencpp (see my previous comment)

This is all way over my head, but thanks for the commentary!

In a nutshell: see #63

and note the referenced patch (amongst other things) implements make install :-)

For future reference:

This library was included in libreoffice in 2019: LibreOffice/core@b4141ca
And it was removed (replaced) in 2021: LibreOffice/core@66e88f4