Well, it isn't even a bill. It is a non-binding resolution - a political stunt. It is, quite literally, a just bunch of ideas they put down on paper. A manifesto. It wouldn't surprise me if AOC and some others got together and wrote a book that went into more detail about what they want. It would carry the same weight as this resolution. A lot of Democrats really like the general idea of what they are advocating for. The idea of doing this is popular.
If this was an actual bill being debated in committee or on the floor of the House/Senate, yes, I agree, it wouldn't have much support. 95% of Congress would either think it is a terrible idea and oppose it or think it is a noble goal but not feasible and oppose it. I totally agree that if it ever became a bill, and got voted on, these presidential candidates would either miss the vote or vote no. But they know it isn't going to become a bill, and the idea of it is popular, so that gives them "cover" to espouse support for it. I think deep down AOC even knows how ridiculous a lot of that stuff is, but she's green and shooting for the moon. Her political capital has never been higher, so she's being really aggressive.