It depends on what you are trying to protect and what options you have if one component fails (replacement).

For highly critical systems that is an issue but it is being adressed. Take as example the space shuttle or the Airbus (3/2) primary/secondary flight computer [1] where the backup is being build by different companies with different processors ...

[1] https://ifs.host.cs.st-andrews.ac.uk/Resources/CaseStudies/A...

By running statistics on a large enough infrastructure you can see how frequently 2 hard drives in the same raid fail within days, or even hours!

Same brand, same batch, operating time, running temperature, even same vibration does trick. :(

We had 3 hard drives (the legendary IBM DeathStars) from the same production batch fail within hours of each other. Part of me was like "Yeah! Awesome quality control!" and a larger part was like "Oh shit." We ended up losing the array and had to restore from backup.

Why?

If the hardware failure is due to a manufacturing problem, then it affects every machine. If it's problem with the production batch, then again, unless you bought from different batches, you haven't reduced risk much.

And if it's a software problem, then again the redundancy isn't helping you.

Basically you haven't succeeded in getting 5 independent tries in the problem -- the failures are highly correlated.

It's the same reasoning that led to the financial crisis -- you have a CDO or an MBS that has a bunch of different obligations so you think you've diversified away risk, but in actuality there is one big factor that causes everything to fail at once.

Also - same shipping way. If I buy four disks from the same shop, then those four disks came in the same delivery to the shop (so they were all dropped at the same time), are handled by the same picker, thrown into the same parcel and dropped a couple times the same way.