That be because Sony and Microsoft (Xbox) were prepared to pay up and commit to big orders.
I’m sure some PC OEMs, as well as Microsoft, could stump up big enough orders if they wanted to to justify some semi-custom work, but only really Valve seem to have (and that doesn’t seem to have been anywhere near as custom).
The SoC actually is completely custom and is console-like for a reason. Here a few reasons. One, the CPU block on the die is a true quad-core Zen 2 meaning it is not seen anywhere else. The die does not have deactivated or fused off cores or it would just be like a Ryzen 4000 APU, an eight-core Zen 2 SoC binned into a quad-core configuration. Two, it also uses elements of Zen 1/Zen+ (power management as I recall, though highly customized), since like the other console APUs, their family design preceded mobile and desktop Zen 2. Three, it is a scaled down Xbox One X/S SoC using the same IP base elements. Four, the SoC originally was codenamed Van Gogh. It was destined to go into a portable Xbox console that Microsoft nixed in the eleventh hour, circa March 2021. Reportedly, Valve took the project elements, ran with it, and the rest is history.
Geez, I forget how titanic this guy’s resume is:
(from the article)
Of course, if anyone outside AMD is qualified to guestimate Zen 5’s performance, it’s Keller. He was lead architect for AMD’s K8 chip, otherwise known as the smash hit Athlon 64.
He later moved to Apple and was instrumental in creating new CPU core designs for iPhones that dominate the smartphone industry to this day and more recently have proven extremely effective when applied to Apple’s Mac computers in the form of the M1 and M2 families of CPUs.
Back in 2012, Keller returned to AMD and oversaw the development of the original Zen architecture. It’s not totally clear how many generations of Zen chips were conceived under his leadership, but it would be a surprise if he didn’t at least have a pretty good idea of what to expect from Zen 5.
After his Zen stint at AMD, he joined Tesla briefly to help the EV company develop in-house self-driving chips and then went on to Intel in 2018. How much of Intel’s recent resurgence with its Alder Lake and Raptor Lake is hard to say. But there’s no doubting Keller is one of if not actually the leading figure in CPU design in the last 20 years.
It’s like these companies just rotate him around, in order to stay competitive with whatever he just made…we might as well just call this whole subcategory “Jim Keller’s Advancements”. 
It’s becoming very clear why AMD waited until now to released dedicated handheld gaming chips.
These have well passed a threshold of not real compromise given the device sizes.
I’m very interested to see what real life battery can be got with the 7040 mobile series
AMD Details Ryzen 7040U Mobile Processors | Tom’s Hardware (tomshardware.com)
