Is the router CPU strong enough to do the job while also cheap enough for a consumer device? That's all that matters.
NAS functionality in a router or network device in general is secondary. Low power consumption and overall cost are primary concerns for the designers.
Chips are like tiny circuit boards, with small traces for electric curent to move between small components in them.
Manufacturing chips is like a CNC, there is a laser drill that etches the electronic circuit and the transistors and whatever else on a special silicon surface.
This laser CNC can be more or less precise, so it can make the circuits smaller or bigger.
Due to how electronics works, what is OK for a particular size of circuit is NOT OK for another. So each CPU design is bound to a specific CNC laser size.
The "size" is called "process node". So chips designed to be manufactured at 14nm (nanometers) process node can only be built by factories that have a laser CNC device that can do that.
Note that this is also not exactly like a CNC, if your factory equipment can make a chip at 14nm, that's all it can do. You cannot take that and use it to make a 28nm or 60nm or whatever just because it is "bigger". So it's very inflexible.
For obvious reasons, older designs are using older (and bigger) process nodes.
The costs of manufacture are related to how many factories there are for a particular process node and how much worldwide demand there is.
In general, older stuff gets cheaper, then there is a cut off point where it becomes more expensive as bigger factories for that process node were shut down and only smaller ones remain.
While I'm calling the factory's tooling "laser CNC" for simplicity, this is VERY high tech and precise machinery, and this means it is VERY expensive.
This is laser machinery that can etch a silicon transistor of the size of a few nanometers. It's VERY small.
Making a new factory for even an existing process node requires more than 5 billion dollars of investment, just to buy the machines, and 3-4 years of setup time before the factory is ready to do anything useful. It's a very slow process that lives of future predictions, because you can't afford to just invest so much money and time like that to realize 2 years later that nobody wants to use that.
This is one of the reasons why there is a so-called "chip shortage" right now and GPUs are so hard to find. There are only two factories in the world that can build the latest GPU chips, and they have been at 100% capacity for at least a year now. There is simply no more available factories to make more of those chips, and "making more factories" requires 2-3 years and stupid amounts of money.
Because they would hurt the sales of their newer CPUs.
But this is economics and there is an important difference between Intel/AMD and most ARM/Mips/Power/whatever else used outside of PC world.
Intel and AMD manufacture and sell a CPU (and chipset and whatnot), a phisical chip. While ARM/Mips/Power/whatever sell the license to use the CPU design on a custom chip, done by someone else (Qualcomm, Broadcomm, Mediatek, Realtek and so on).
As I said above, the costs to design and manufacture actual hardware are VERY high, so you cannot afford to keep making older chips that are still pretty decent and will significantly decrease the sales of your newer stuff.
But ARM/Mips/Power/whatever don't have these costs. They have made a blueprint, a circuit design for a CPU. So for them it is still profitable to keep selling even very old CPU designs to other companies.
And the companies that actually make the chip don't care about CPU design age or anything like that. They are making a chip for a specific usecase.
For example a wifi router, or a media center, or a smart TV, or a businness firewall appliance, or anything else that requires a digital CPU.
The CPU is only one of the components in that chip that is called "SoC" or "System on a Chip", there will be wifi, ethernet switching, pcie and USB controllers, and other types of components for specific applications, that are also a very important selling point of that product.
As long as their design is good enough to do the job they are selling it for, it is good for them.