A hard drive may fail if a single piece of the smallest dust, or even fingerprint particle is present on the disk, so it is well secured in metal cases.
A CD, or DVD (or any other optical disk) is stored casually in plastic cases and may even stack together with other CDs, indicating that it is very dust-proof. We only need to wipe it with wet paoer tissues and then let it dry before we can put it into a DVD reader to get its content.
Why is the difference?
There are a few reasons including - Density of the disks - a hard drive has a much, much higher density then a DVD. Different media properties - a DVD is not designed for speed, nor does it use magnetic flux. Similarly it is not really a read-write medium (although it can sometimes be used a bit like one) Error correction - DVDs have a lot of redundancy to handle things like scratches.
It's worth noting that very old Hard drives had much greater tolerances, and you could remove the platters without a cleanroom. As densities increased, tolerances decreased, and those days are more-or-less at an end. (It's still possible to do sometomes, but has a high failure rate)
For optical disk a laser beam is not focused on the surface and this is by design. There is a transparent layer that refracts the beam further so it's focused inside the disk where data is written. Particles outside this layer that are tiny enough may not eclipse the entire beam.
When the disk spins, the particle stays within the beam for some time, affecting multiple readings but (hopefully) every single reading is not affected enough to matter. If the data were on the surface and the beam focused there, a particle almost as small as a single pit or land might eclipse the beam entirely.
Tiny scratches undergo the same rules.
Similar phenomenon allows you to use glasses with an eyelash stuck to one of the lens. The eyelash is not if focus, it blurs in front of your surroundings. You may notice it's there but it doesn't entirely cover any point of your view at any given time.
Optical disks also use redundancy to perform error corrections. Without this redundancy read errors would manifest frequently. Compare this answer of mine: CD-ROM subchannel is different when dumping the same disc.
A hard drive needs its head to be very close to its platter:
In 2011, the flying height in modern drives was a few nanometers. Thus, the head can collide with even an obstruction as thin as a fingerprint or a particle of smoke.
I agree with sawdust's comment:
The HDD requires the read/write head to literally fly above the platter surface as close as possible so that the electromagnetic coil can either get a current induced by the flux patterns on the platter (during a read), or write a new flux pattern using minimal energy so as to not disrupt adjacent tracks.
