The truth is that those are just niche items. Having a need for both a Linux-grade CPU and a real time MCU-like core is not that common. It tends to show up in projects too advanced for most to attempt.
And if by "very easy to work" you mean "just allocate a shared memory block and put a mailbox in there for the Linux userspace and the RT MCU to talk to each other", you may have inflated expectations of what an average developer using an SBC can easily do. It's "very easy" in the same way rolling your own PCBs or writing your own drivers is very easy. Seasoned developers do it without thinking twice, and then stand dumbstruck when they see entry level developers avoid that like the plague.
Not like we're lacking for the offerings. Plenty of modern SoCs have MCU-like real time cores in them - intended for low power operation, real time control, or both. Even the mainstream RPi 5 boards ship spare PIO and MCU cores - in the RP1 chip. The vendor just couldn't be bothered to document that.
There’s whole sectors of industrial and automotive using these AMP SoMs. And relative to these RPi only recently began providing options that you could design into an embedded product at all. Their FAE and supply presence is way behind.
The boards that people reach for when they roll their first projects, the hardware that ends up in early prototypes and one-off devices, the hardware that gets targeted by open source projects. The chips that return more than a "databrief" 8 pages long and a single page on a vendor site inviting you to visit the snake pit with "contact sales for more information". The boards you can actually buy with a fixed price tag and a delivery to door, and can find writeups on how to use them for specific things.
If you know what "FAE" is, let alone can get into a meeting with one, you're deep in the trenches, and are pretty much expected to wrangle an extremely poorly documented MCU core that happens to be shipped in a SoC as an afterthought into submission.
Those are just the ones off the top of my head. I'm sure there's more if I used Google to search. And of those companies, there's *multiple* lines of chips that offer these capabilities.
Its... pretty mainstream in the embedded world. The fact is that Linux is very bad for realtime, so adding a non-Linux coprocessor is one of the easiest ways to solve the problem. Run Linux for "regular" code, and then run the hard-realtime code on a dedicated chip (either Cortex M4 or some other MCU... or maybe a Cortex-R(something)). Or in the case of Xilinx, run the realtime stuff in FPGA-land.
Alternatively, you spend a ton of money on an OS that's well suited for hard-realtime (ex: VxWorks or something), but that also requires a processor with some guarantees (usually L1-like cache that's reserved by the programmer, some "always active" hard-realtime memory). This is more difficult in practice compared to just having a 2nd MCU or FPGA handle the realtime tasks though in a coprocessor like arrangement.
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I dunno what you're arguing for. Are you trying to say that the world of Linux capable core + Realtime core is somehow... niche?
Engineers actually use these in real products and embedded systems.
The combination of Linux and microcontrollers isn’t new. It’s a very valuable and common structure.
The comments here trying to dismiss it or call it uncommon are bizarre.