Just as when driving a car, where it's a good idea to check the rearview mirrors every 20 seconds or so, PM and PF should glance at their PFD regularly.
With the tape displays and everything on the one screen, one can assimilate whether the situation is normal in about a second. It is very easy to check if the speed, atttitude and altitude are where they should be with just one glance.
I suspect the problem stems from pilots no longer working their way 'up' to big jets as we used to, via small and then large turbo props, but going from flight school straight into the RHS of a modern airliner that seldom goes wrong.
Such pilots perhaps do not develop the vital need to monitor because the modern jets' automatics rarely go wrong, whereas those of us who flew aircraft with crude and unreliable 'automatics' (or no automatics) quickly developed a good regular scan to check that all was well, or to do something about it if it wasn't.
This ties in with automation dependancy.