Why were there no SMART warnings?



My external USB hard drive suddenly started clicking and was no longer accessible. There had been no SMART warnings from HDS. I sent the drive in for Diagnostics/Data Recovery, and was told that the magnetic media had been slowly deteriorating and a head had now crashed.
Why were there no SMART warnings?

Please help.

I didn’t find the right solution from the internet.

Professional Video Production Studio

Thank you