What do you mean by disappear? Did Joe Biden disappear after Trump was elected only to show up and run for President? No it's just that he didn't have a job in politics so the media didn't cover anything he was doing and he "disappeared". If it wasn't for the medias obsession with trump he probably would've disappeared in these last few years too. What do you expect a candidate whose not currently in office to do between runs that wouldn't make them disappear?

She should work on building her party

She does, but this isn't England there's no mass party system where there is much off election work to do. If your party doesn't have any representation then it's job is to get representation, and the only time you can do that is during election periods, especially big ones like the presidential season. Even party's that have representation do most of there work during the campaign season. The Democrats ramp up there work and engagement multiple times over leading into the election.

Tear impressionable young left-leaning voters from a party that could win

Kamala's Gaza policy did that, stein is just picking up people who wouldn't have voted any way or would've left President blank. The people voting for her aren't stupid, they know she isn't going to win, they're voting for her because they don't like Kamala and are trying to send her a message.