Well I remember playing GTA Online on my 12Gb PS3 Ultra Slim and it was fine
And the ps3 version was much smaller. They've spent the last 7 years adding more and more content to online and the file size ballooning. And I assume you'd want higher quality assets than PS3....
That's not to say they couldn't release it on Switch, but even with a 32gb physical cart you'd be looking at a sizeable 10-20gb install to get all the content of the other versions.