Because you have to start somewhere. If Apple entirely guesses wrong and the little 4K box becomes completely obsolete when some complete standard is finally realized, is that really a big deal? The box as is costs $99.
Apple just embraced USB 3C before much of the world has jumped on it. While that may be more of an official standard than 4K at this point, there's little available for it now. However, Apple jumping on it may make it a standard that gains some legs.
If Apple best guesses a 4K standard for

TV, it's likely they'll implement the same standard in 4K recording of next-gen iPhones and iPads. If so, that will lead to tens of millions of devices using Apple's choice of 4K. In short, Apple may have enough power to best guess at a probable standard and actually make it a standard. Look at the lightning connector as a good example.
How much better is Skylake than current CPUs?
How much better is 5G than 4G?
How much better is the A9 than the A8?
It's always that way... in all tech. Apply that same argument to everything out now and there's little reason to upgrade from anything. There's not an app in the app store today that can take advantage of anything special in the A9. So why bother developing iDevices with A9s or Skylake Macs? Etc.
In tech you build the hardware and the software then plays "catch up". If you've seen 4K television displays (showing 4K sources), it's obviously sharper than 1080p. It doesn't take massive new TVs- it looks sharper on "regular size" TVs.
How much better is retina in 4"-5.5" screens? Some would argue it's far, FAR better than non-retina. Maybe it is. In the latest round of updates, Apple went beyond what was defined as retina. So was regular "retina" not the max that human eyes can resolve? Was that just marketing spin? Or is the new retina in the latest iDevices overkill (beyond what human eyes can resolve)? Whatever you or I believe, Apple sure sold a LOT of iDevices, both when regular retina was the "next big thing" and when the latest upgrade to retina was a "next big thing".
Does a 5K iMac matter? It's the same 27" screen size as the non-retina iMac? Where are you arguing against a 5K iMac?
I've seen "the chart" argument done to death. I've read the arguments about distances from screens, average Joe's lighting, etc. But in the end, 4K is TVs version of a 5K iMac or 5.5" retina iPhones or Skylake or 5G or A9. Each time I look at 4K screens, I think I see much sharper pictures than I can replicate on my 1080p set. Maybe my 1080p TV is just poor quality? Or maybe 4K resolution even on the same size screen IS visibly superior to human eyes.
Whether true or false, the industry is marching there. Will there still be 1080p TVs for sale in 2 or 3 years. Can you find 720p sets for sale anywhere now? Some of these same kinds of arguments used to be slung when Apple was locked in on 720p as their MAX resolution.
I can certainly see an argument against Apple going 4K in a next-gen

TV. Then again, while a very limited sample, it looks like Apple gets around to upgrading the little box about once ever 4 years. If I'm right about there being no 1080p TVs for sale in 2 or 3 years, the next

TV would be seeming pretty obsolete at about the mid-point of it's "usual" upgrade cycle... just as Apple clinging to 720p in a 2nd gen

TV made it one of the last bits of new hardware for sale when most everything else in the HD market had pretty much fully embraced 1080p.
Do they need to go 4K with this one? No. But why not? Go 4K and they can be a bit ahead of the curve. Better hardware can play current 1080p and fading 720p and SD at it's MAX. Stick to 1080p and that won't work the other way.
Does Apple need to go A9 or Skylake or 5K retina iMac? No. But why not? Many of us whine when Apple doesn't roll out new computers with latest & greatest graphics card (even if no Apple software can currently max it out). But then, with this ONE thing, some of us seem to default to the "good enough" argument and/or the "until every other part of the equation is fully resolved or in place" argument. Why do we "think different" with "good enough" with this ONE thing... when we whine for bleeding edge in everything else Apple makes?
If someone can't see much difference or is completely happy with 1080p, a 4K

TV box doesn't force them to go 4K at all. Their 1080p or 720p will play exactly the same on better hardware... just like their favorite iDevice apps will run just fine on a new A9 chip. Just as one can rent their choice of 1080p or 720p or SD in the iTunes store now, any 4K content added to the store would almost certainly come with the alternate choices of 1080p or 720p or SD too. In short, there's no consequence for any anti-4K consumers should Apple roll out one capable of 4K. This piece of hardware will just have some untapped capabilities beyond what those users do with it... just like there's likely untapped capabilities in our current Apple hardware because we simply choose not to use it too. For example, there's a LOT of people out there with iPads with cellular capabilities who rarely- or even never- use the cellular hardware. It's there. They could use it if they want to do so. But they choose to use wifi only or tether from their iPhones, etc.