Several facts lead to this:
1. No one wants to provide support to non astute users calling up Tech Support saying their SSD is full.
2. Proponents love to quote benchmark results but people don't use computers to run benchmarks.
3. People see the world change when they go from an old, slow 5400 rpm HD to a new SSD. To their eyes, the SSD is responsible for 100% of the change, not the outdated hardware, not the slow rpm, not the cluttered 4 year old OS install filled with bloatware. On my kids boxes, I wiped the drive and reinstalled the OS every Xmas vacation. Now that they are all educated users, that problem no longer exists. But educated consumers don't but PCs off the selves at BestBuy or Walmart
4. Testers tend to take the user out of the equation .... their "aha moments" tend to become irrelevant when the user is part of the "time to finish" measurement. A test might involve a script of 27 CAD functions and the test times it and says "aha, see the SSD finished in 45 seconds, the other thing took...". With a user involved, after an engineer takes a two red pencil marked up drawing to two CAD Operators:
a) CAD 1 guy clicks on the file to open up the Drawing ... it takes 0.5 seconds
b) CAD 1 guy clicks on the file to open up the Drawing ... it takes 1.0 seconds
c) Both guys are reviewing the markups to make sure they understand everything .. it takes 22 seconds before they look at the screen again and are ready to work. One was "twice as fast", but both will finish the same task in the same time.
d) As they complete each edit, there is no "productivity increase" to be obatined as the user is the slowest link in the chain.
5. Here's our boot testing on a single box equipped with two Samsung Pro 256 GB SSDs, two 2 TB SSHDs and one 7200 rpm HD
HD - 21.2 seconds
SSHD - 16.5 seconds
SSD - 15.6 seconds
Now when we told users tho try and identify which device they were booting from. most could recognize the HD as "noticeably slower", no one could tell the difference between the SSD and SSHD.
And boot time really doesn't matter when peeps come to work, sit at their desk, boot their machine and then walk away to get a cuppa java ... or, like today... discuss Game of Thrones for 20 minutes.
We also switched boot order in the BIOS without telling anyone day to day ... with a copy of the OS on all three storage devices. So when folks booted from HD, SSDS, SSHD, no one said ... "Gee my computer was slow today". Also tried testing how long it took for me to be able to move my toon in an MMO ... 44.5 seconds with each device, the handshaking and downloading info from the server was obviously the limiting factor here.
6. I use the "test box" on a daily basis and "normal boot" is from the SSD with OS and programs. Its a water cooled SLI box and the inclusion of an SSD for this purpose was not a "budget" consideration nor was a 2nd SSD for "fav games". After not playing Witcher 3 for a while, latest B&W DLC was released and while waiting for loads I was telling myself ... "I really should move this to the SSD". Went I went to do so, it already was on the SSD.
7. We also had two laptops built (Clevo) to our specifications for use in the field (CAD workstations) on job sites, which of course peeps loaded and played games on on their own time. They were set up for use by several folks basically identical except for one thing ... one had an SSD + 7200 rpm HD and the other had and SSHD. No one had any comment about any difference in performance between the two lappies.
So while it is inarguable that SSDs are faster than SSHDs and both are substantially faster then HDs, the simple fact is that, for most users, the people buying "shiny new PCs" form Walmart or BestBuy will not be impacted in any significant way, certainly not going from an SSHD to a SSD. That's a long way from the individual getting a new CAD Station, Video Editing Workstation or even "killer gaming rig".
For both laptops and desktops, anyone who will let the customer choose options, even the lowest cost ($889 in example below) offering provides choice here:
https://lpc-digital.com/product/sager-np5652-clevo-w650rc/
With a 1 TB HGST 7200 RPM SATA 6Gb/s Hard Drive as the "base offering", options are:
SanDisk X400 256GB SSD SATA (6Gb/s) +$55.00
1TB Seagate Solid State Hybrid 8GB SATA 6.0Gb/s Hard Drive +$50.00
250GB Samsung® 850 EVO Series SATA 6Gbps SSD +$70.00
2TB 5400 RPM SATA 3.0Gb/s Hard Drive +$70.00
256GB Samsung® 850 Pro Series SATA 6Gbps SSD +$105.00
SanDisk X400 512GB SSD SATA (6Gb/s) +$120.00
500GB Samsung® 850 EVO Series SATA 6Gbps SSD +$150.00
SanDisk X400 1TB SSD SATA (6Gb/s) +$245.00
SAMSUNG® 850 EVO™ 1TB SATA III 3-D Vertical SSD +$335.00
SAMSUNG® 850 PRO™ 1TB SATA III 3-D Vertical SSD +$440.00
SAMSUNG® 850 EVO™ 2TB SATA III 3-D Vertical SSD +$695.00
SAMSUNG® 850 PRO™ 2TB SATA III 3-D Vertical SSD +$920.00
8. The stores that sell these "shiny new PCs" are competing on price. Including a SSD raises price and costs sales.
As a result, most of the folks shopping for "shiny new PCs" don't even know what an SSD is ...
... these are the folks who are asking salesman of the computer has enough "memory" to store all the grandkids photos.
... these are the folks who are not going to have a clue about saving a file to a HD or SSD.
They don't include them because they know it doesn't help move units...and it has a downside when these folks start calling TS cause they can't figure out how to manage storing files in different locations.