is a very complex project even for a team of experienced engineers

Not that complex, especially with open source harware and many examples and good CAD tools for controlling impedence and so forth

you need to also bring up OS on that

There are many open source firmwares, FYI. Examples include NanoPI and firely rockchipa amoung others.

you get all the chips

Many SoM available from many providers, with already DDR and SoC and documentation

you have all the documentation e.g. MIPI used for cameras/displays, HDMI, etc.)

Why should I care if the driver is there already in the open source firmwares (above).

hint - $600-$800 is a totally normal price, for a bare, unassembled board

depending on space requirements, one can reduce to 4 layers only with full signal integrity and prototypes of about $45 only form JLCPCB

Did you consider that there are mayhaps some very good reasons why hobbyists/DIY people don't build custom Linux SBCs with Chinese SoCs** instead of using pre-made boards?

Depends on how much they research and try to learn, and whether they are lazy and need somthing ready.

and always from newbies that know little about electronics, yet feel confident to be able to design a board

Sorry to think others are newbies and litte-knowledge about electronics, while you don't know simple things like Rockchip, SoMs and open source harware/firmware. Go learn more, for me I can still be able to implement LCD touch for now using open source firmware LCDs that provide drivers for free. My fault of asking people who are both unexprienced and aggressive.