Even if you just want one platform, there is a lot of machinery you need even to boot up, load something from disk, access a USB keyboard and mouse, and display some text. ACPI and UEFI add considerably to this.

Then there's bugs and debugging. If you have a bug in your disk driver you can slowly lose data without noticing. Quite a lot of other bugs will lock your system and you need to debug painfully from whatever logs you can find. Building in a non-C language might make this a bit easier but won't save you from the hardware locking up.

Once you've done this, what have you gained? You have an OS with no userland and no applications. What bits of Linux/BSD do you actually want to replace with something else? The code is fairly modular and OSKit exists to help with this.

The nearest to what you describe is probably retrocomputing; people building their own considerably simpler systems from the ground up. If you want a simpler platform to start on without having to first build the platform, consider Raspberry Pi; the bootloader there effectively just pulls an ARM ELF executable off the SD card and starts executing it. Have a look at https://github.com/dwelch67/raspberrypi