Is this really true? The computer ecosystem is more open now than ever. The original PC BIOS (which PC-compatible manufacturers needed to implement) was never an open, documented standard. It was a proprietary, closed system made by IBM. It's pretty fair to say that IBM didn't anticipate a PC/x86 ecosystem developing around their product. They even sued companies who made their own compatible BIOSes (like Corona). Intel didn't really have much to do with the success of the product at that point in time either, much less Microsoft.
In contrast, every widely-used modern system for hardware abstraction (UEFI/ACPI/DeviceTree/OpenSBI/etc) are open, royalty-free standards that anyone can use. Their implementation in ARM is newer, and inconsistent, but that's only because of how hugely diverse the ARM ecosystem is.
On the other hand, ARM sells the cores to SoC vendors (and doesn't care much what becomes of it), SoC vendors ducktape the ARM cores to a bunch of Synopsys peripherals and sell the resulting SoCs to smartphone and car makers (and doesn't care much for the product). System integrators throw Android on top and sell it to the customers. Then Google, who get all the cream via Play, hides all the mess behind a thousand layers of Java abstractions.
DeviceTree is an offshot of Sun's OpenFirmware (and it leaves out all the hard stuff - OpenFirmware had Forth, DeviceTree expects the kernel to support every single brand of fan switch). OpenSBI is a disaster. I'm sorry, but what kind of bright mind came up with the idea of hiding damn *timer* behind a privilege switch? Timers were enough of a pain point on x86 already, then it settled on userspace-accesable RDTSC. RISC-V SBI? Reproducing x86 one stupid decision at a time.
I think the issue is that desktop and server computing are “open” in the sense that you have full control over the software you run on them. So people interpret the dominant desktop and server platform architecture (the world of x86-64) as being open.
The embedded world is mostly closed, you are meant to run the software your hardware comes with. The platform’s popular there are considered less open (ARM and RISC-V).
Mobile devices like phones and tablets are historically closed devices, regardless of ISA. They are generally getting more closed in the name of security.
It is not the ISA that is “open” but the industry.
That said, in RISC-V, there is a sub-current of openness. I do not think that will overcome the industry tendencies in general, but there will be a small cadre of folks trying to create an open presence in every niche. The good news is that there is nothing to stop them. They will succeed eventually.