quickduck I know other ROM, forgot its name, is selling refurbished phone with OS preinstalled. Why GrapheneOS doesn't do such thing?
Because they're not a for-profit company, and I imagine logistics and other stuff would be too much additional strain on resources and such. There are however stores that offer pre-installed GOS phones, e.g. Nitrokey (sorry in advance mods if this is against rules).
My browser is Firefox and I disabled USB thing (webUSB is too dangerous attack surface) and not wiling to enable it
I don't believe Firefox even has (proper) webUSB support, does it? I was under the impression it didn't.
The install steps requires extreme terminal work which I do not understand (I said I do use Linux, I know what terminal is and I use it, I just don't understand the full view)
Then just use a supported browser and install with the web installer. Personally, I don't see much reason to spend (exorbitant amounts of) money to avoid self installation, but if you want that, then you can check aforementioned stores.