Why do hard disk drives mostly have the same speed? They are mostly 5400, 7200, 10000 RPM. Why not 6000 or 8000? Is there any reason or benefit for having those speeds?
The spindle rotation speed of a hard disk depends on a spindle motor, naturally. Since there are only very few manufacturers of these motors, the available rotational range is limited. In fact, the current global shortage of hard disks is because of two flooded hard disk motor factories in Thailand.
The most common rotational speed was 3600 rpm in the 1980s. This is because oscillators and motors that are based around 60-hz are very common. Standard North American AC mains frequency is 60hz. 60hz translates to 3600 RPMs. Therefore 3600 rpm AC motors are widely available. Naturally, the designs that followed were based on the same rotational speeds/frequencies.
So the most common speeds ever used are 3600, 3600*1.5=5400, 3600*2=7200. Faster drives use 10,000 and 15,000 rotational speed, apparently they use different frequencies.
This StorageReview article discusses spin speed in depth.
Right now, you can get drives as fast as 15K. We're unlikely to ever see drives spin any faster, for two reasons:
When rotational velocity remains constant, linear velocity increases exponentially as you get further from the center. This is why all the 15K drives you see tend to come with 2.5" platters (the link above being a notable exception). They could make a faster drive if they reduce the platter size (and therefore capacity), but we're unlikely to ever see this because of item #1 above and because of motor issues outlined in other answers.
For the rest of it, I think the real answer has more to do with marketing. Sure, there are specific models of motor out there, but if a manufacture really wanted to market a 7500 or 8000 rpm drive, I expect they could work with their suppliers to make it happen. It's more believable to me that they find it easier to sell drives when there are just a few varieties that are well-understood by consumers.
That being said, the reason there isn't thousands of different ones is because it isn't likely that a 7200RPM one goes at exactly 7200RPM - these are the average speeds of the motors.
In addition, as much as there are only a few hard drive vendors in the world, there are even fewer specialist vendors who can produce motors of a quality and quantity needed for hard drive production. Because of this, why would they want to make 20-30 different motors instead of specialising/mass producing just a few.
However, there have been a few newer speeds creeping in such as 5600 and 5900 drives in recent years which are aimed to be "greener" than 7200RPM whilst maintaining good speeds.
As for why we don't have faster 100k drives.... All I can say is physics and thermodynamics! If it was possible, we would - but, generally, the faster the speed, the lower the capacity - because, you have to remember, it isn't just about the speed the disks spin at, the read head has to be able to keep up and read over the platter, in addition, the faster the spin, the higher the heat.
All this being said, I don't think we are going to see many more traditional hard drive technologies - give it 2 years and we are probably just going to have slower green disks for NASs/mass storage and SSDs for mainstream.
(And thankfully, the 4800RPM laptop drive is nearly completely dead!)
If you look at the situation from HDD manufacturers' point of view, the reliability, the speed, etc. of drives should be tested separately. So the more variety of speeds you have, the more it increases your general expenses. Note that common HDD speeds are listed as 3600, 3811, 4000, 4400, 4500, 5200, 5400, 7200, 10000 and 15000 RPM (the first IBM hard drives actually spun at 1200 rpm).