121.5 doesn't need to be continuously monitored.
Not during engine start, taxi and all that other good stuff.
You want to monitor 121.5 inflight in case you cock up a frequency change and ATC is trying to reach you.
When nothing else is going on you monitor for somebody else's problems.
But international crews might be jabbering on all three radios at once.
Two FIR crossings with no handover and the third to keep the current controller happy.
Radios are tools, use them.