Flying an airliner by hand at cruise is dangerous and usually constitutes an emergency. ATC expects the plane to stay precisely at its assigned altitude, especially in RVSM airspace. It's difficult to keep that altitude by hand at speed. Cruise is also normally done close to the edge of the plane's abilities, so small deviations can have catastrophic effects (overspeed or stall).

Pilots routinely do climbs and descents at lower altitudes by hand to practice their skills, but not cruise. See this Airbus doc: https://safetyfirst.airbus.com/high-altitude-manual-flying/