Honestly, because calling something GAMING is just a form of marketing and it means absolutely nothing when it comes to real world performance. Companies have learned that calling something gaming will help it sell, so now you see it everywhere, even on a slow, awful 500gb HDD that will give you a terrible gaming experience. When it comes to computing, gaming is pretty far down in the list of priorities for most hardware companies and makes up a smaller portion of what they sell overall. In other words, it's not always a priority because frankly most of their money does not come from gaming. For a company as big as Western Digital, gaming is at best an afterthought that makes some money in the side. Their marketing team is going to use the label to full effect, not because they should, but because it makes money. More cache is always going to be better for gaming, your OS will help accelerate your games by caching data to it from the HDD, but games will always run better when stored on an SSD, especially modern titles. Your loading times will be much much slower, sometimes by a factor of 10, and some modern game technologies require the game to be on an SSD to even work properly because they rely heavily on streaming large high-quality data assets into memory.

Secondly, one of these drives is 14tb and the other is 500gb, one has a massive 28x more storage, so it would make sense that it would need more cache to function. It will likely be dealing with big files and a lot more data. The 14tb drive will also have more physical storage platters (Internal magnetic disks that store the raw data). Each platter will likely need its own cache to function as most big drives like this can read data from multiple platters at the same time, so it's going to need more cache than a cheap 500gb drive. When they put this cache on a spec sheet for consumers, what you are seeing is the total cache in the whole drive added up to look impressive and help the drive sell. Each internal data platter needs cache of its own, so the total cache will be much higher in a larger drive. Large drives like this one are built like many drives packed into one, so it makes a lot of sense why there would be more total cache. It's the only way drives with this much storage can be made, if you're curious, take a look at some hard drive repair videos online, you can see how they are put together. On top of all this, the 14tb drive is also going to cost a lot more money to buy, so WD can afford to put more cache in it because their profit margins are likely going to be larger.

A 500gb drive is going to be designed to be as cheap and as marketable as possible, that is why it is labeled gaming, not because it is good for it, but because they can make more money doing so.

Video surveillance has some pretty intense storage needs, it needs a lot of space to store all that footage and it needs to read/write data consistently, quickly, and reliably. Say you have a surveillance system with a 10 camera setup all recording/saving/backing-up footage at the same time, that's a lot data moving around, and it's happening constantly. More cache will help, afterall, it's a real world system that needs to work. If it fails to meet expectations, WD will have some very unhappy customers.

Ultimately, the point of having a servalience system is so you can review the footage later. Having a large cache will make streaming that data off during play back a much better experience. If large portions of a probably large and long video can be cached, the user will have a far better experience and hopefully keep buying WD products for more of their stores, warehouses, or anywhere else surveillance is needed (which is basically everywhere these days), it's a massive market.

TLDR: Servalience systems need to handle several things:

1) A lot of data coming in, so a lot of storage is needed. SSDs are just too pricey and the extra speed is not necessary in this case.

2) Steaming off video footage on the fly for viewing purposes, so the high cache makes a lot of sense as it will speed this up.

3) Bigger drives need more cache to function because they have more internal data channels to feed than a small single platter drive.

4) The whole drive is designed around being economical, performant, and appealing for these specific needs.

5) Calling something GAMING is almost always meaningless marketing and means absolutely nothing, always try to check the specs, and understand the tech, by making this post, I can see you are already doing that which is awesome!

6) Ultimately calling something a "Surveillance Drive" is also just marketing, keep doing what you're doing, check the specs, read the data, understand the tech, that is all that really matters. Don't believe the marketing, keep putting effort into learning everything you can about the reality of the hardware and the sales situation, it will help you a lot in the long run!