Because it’s “quick start”. Least effort to get a taste of it. For actual deployment I would use compose as well.
Many project also have a example docker-compose.yml in the repository if you dig not so deep into it
There is www.composerize.com to convert run command to compose. Works ~80% of the time.
I honestly don’t understand why anyone would make “curl and bash” the officially installation method these days, with docker around. Unless this is the ONLY thing you install on the system, so many things can go wrong.