Why doesn’t it? There are 8 million unemployed, plus 6.5 million who haven’t looked for a job recently enough to qualify as unemployed but who say they want a job, plus 6.5 million who want full-time work but only have a part-time job. Right there you’ve got 21 million, and there’s another 20 million or so adults aged 25 to 64 who are not institutionalized and not working. They’re not all disabled, and differing economic situations may well attract into the workforce people who are currently sitting it out for various reasons (schooling, stay-at-home parents, early retirees, etc.).
Why is it impossible? Advanced economies typically don’t grow that fast because of a lack of excess inputs to fuel growth, but Bernie is planning on pumping a lot of extra resources into the economy. It’s the same theory as stimulus spending: the 2009 stimulus, e.g., is credited as having added perhaps two points to GDP. Why would a similar but larger stimulus not add several points to existing positive growth?
Income growth, by itself, doesn’t necessarily cause inflation. It depends on what goods and services are available for sale, and surging productivity would make a lot more stuff available.