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This is normal for certain kind of libraries, such as C standard library. This is why you want Valgrind suppression files, so it knows which leaks are normal. To easily have nice suppression data for Qt, run Valgrind under Qt Creator (documentation link).

The reason for this is, libraries reserve memory for things like communicating with the operating system stuff (such as windowing system), but have no control over when your application no longer needs it. It could be released at application exit via exit hooks, but the entire application is going to be removed from memory, so doing memory cleanup would just slow down application shutdown for literally no gain at all.

Note that most memory is still cleaned up as C++ objects go out of scope. C++ objects need to have their destructors called anyway, which is normally done by delete for objects in heap. Also this is definitely not something you should optimize for in a normal C++ application code. The stuff libraries leave allocated at exit (and need Valgrind suppression) are typically low level stuff, global to the whole application and not owned by a single C++ object instance.