She may think with such a crowded field, there will be a brokered convention.
In a brokered convention, with no first ballot nominee, anything can happen. A split convention could nominate Hillary as a compromise candidate. In that case, Hillary would be the nominee without having had to campaign up to that point.
You don’t have to be a candidate in the primaries to be nominated for president. She may hope something happens that the Democrats turn to her as their savior.
In theory. But it's highly unlikely that something like that would happen.
In the olden days, when things like that happened, it was because candidates didn't really run against each other.
They each had a block of delegates committed to them show up at the convention and slug it out.
Nowadays, they slug it out in the primaries and the weak ones withdraw early on.
I think she's leaving the door open because if she closes it completely, her audience loses interest in her fast.
I think the supporters of the other candidates would be enraged, just like the Bernie Bros were in 2016 in response to the Wicked Witch rigging the nomination process. Having her get the nomination after winning no primaries or caucuses would be immensely stupid. Hopefully, the Democrats are that stupid.