Every one of them out there right now or in the past is flawed in at least one or more ways. So why hasn’t a company built something that does everything possible with a sample yet? Surely we’d all want one, even if it was quite pricey 
(Not counting DAWs, something in hardware)
If you can define ‘perfect’ in a way that everybody agrees upon, you’ll be a rich man
I think there’s a lot of customers out there for a sampler that combines the best/removes the worst of the current crop, but as Braken says, ‘perfect’ is impossible. Too many different use cases.
Well when it comes to just a sampler, what doesn’t the Octatrack do with a sample? I get there are things so far as say you don’t get polyphony per track, but each track can be a note, and you can create chords etc.
I’m sure there are some mangling or features of sorts I know nothing of in software and yes better time stretch. But what more can you actually do with the sample it self? So far as the whole groovebox DAW in a box type of idea there are too many factors to please everyone.
According to?
Because they’re too busy building stupid ugly battery handles? 
Everyone would have a different idea, as has been said. And even then, the best keyboard sampler wouldn’t be the best groove box or sampler-mangler.
With that in mind, the Akai Z4/Z8 was the closest I’ve ever seen to a perfect multisampler in a rack. It might not have been a perfect beat-making sampler, but as multi-samplers go, it was the most sophisticated and best-sounding I’ve touched. 24/96 stereo sampling, built-in dual effects card, optional hard drive, multiple outputs (with more optional) and a USB port on the front. Plus a nice software editor. All in 2002. I so wish the MPC’s had the same level of editing, sound quality. Don’t get me wrong, I think the current MPCs are fantastic - I own a Live 2 - but I also still have a loaded Z4. That thing was awesome.
I’d second @Scot_Solida - if you added just a couple more ways to connect the LFOs or CCs to sample start/loop params (Ensoniq-style) the Z4/8/MPC4000 series would probably cover anything you might want to throw at them…
Otherwise, the ‘perfect sampler’ is something you spend a decade developing in Chuck or Csound : )
I think a better word would be room for improvement since nothing is perfect. Current tech, keeping cost down, not ballooning in size and specs which the majority won’t use and software talent at the company etc. have a part to play.
why has no one built the perfect guitar yet?
Multiple grains, proper polyphony etc etc.
Someone always says something like not enough outs, doesn’t do stereo, FX are garbage, doesn’t have granular, the list goes on
Yes, the closest to perfect yet, then
Yeah I never listen to them… they dont know they they’re born .
the people that think it’s already perfect are too busy using it to complain about it.
I guess another way of putting it is why not super high resolution for supreme quality when needed, other modes for low-res 12-bit, and just give one device the best of everything seen in the past and present in terms of different ways of manipulating a sample. Granular, wavetable, etc, all in one device that isn’t purposely hampered in some way. It seems that so many companies have great ideas but fall short in critical areas, making finding the ‘perfect’ sampler almost impossible. And yes, I do think Elektron are probably best placed to produce such an ‘ideal’ sampler because of the sequencer.
Yeah, I knew there were going to be features I knew nothing about. 
I remember reading Sound On Sound reviews of those and lusting after them. They were out of my price range back then tho. And as you said, that was 2002. So with today’s technology, manufacturing processes, and computing power, it doesn’t seem out of the realm of possibility to build an everything and the kitchen sink sampler imo, especially with hardware being so en vogue nowadays.
(edit sry - re. Akai Z4/8) One feature is disk streaming - they never ported over the disk playback from the SXXXX range to the Zeds. Apparently streaming is something that most people can’t do without.
new sampler ?
proper polyphony and multitimbrality (more than 8 mono / sterio samples)
slicing on grid/zero/transients.
pitch detection.
lots of modulation
stereo / binaural / dolby/7 outs or whatever is coming up
midi 2.
usb / lightning/whatever is new … midi and audio
streaming from disk, usb, bluetooth, ethernet, wifi, 5g
ios/android app.
flac,mp3,aiff,raw,wav etc.
lots of filters, old bitrates, machine emulations.
does whatever kontakt can do as it seems popular.
ability to blend/enveloper blend on a mono track (so you can define how one sample cuts off another ,… perhaps saturate/volume/an very quick transient style envelope to stop the instand cut.
crazy types of loop points.
mpe.
SSD.
costs under 1k
lots of knobs,encoders,buttons.
xox or normal sequencing(thgouh its probably not so important as theres many external sequencers).
firmware that isnt full of bugs.
screens that dont ‘burn in’
a plug that matches the territory that i’m in
*uk spelling of color / colour .
*correct pronunciation of aluminium
*something thats ‘coming home’ or whatever jinogistic rubbish is floating around society at the time
*optional.