Why haven’t undocumented dreamers gone though the legal process of becoming us citizens?

They have to return to their country of citizenship first! They must obtain a legal visa or have a petition filed on their behalf by a qualified immediate relative, after they complete their bar to reentry. There is no way to change status of anyone without any status, i.e. an illegal alien. IF these delusional "dreamers" had returned to their country of citizenship before they turned 18, they would not have been penalized for their parents' illegal acts, and they could have applied immediately for any visa they qualified for. Instead, most "dreamers" chose to remain illegally, so it is no longer a matter of "may parents did this to me," but their own adult choice to break the law. Now they go home & complete bar to reentry. They might have a chance to return legally in the future, but their record of violating the law will always be against them. Their own fault, and nobody else's, if they chose to continue breaking the law.

Because they can't 

There is no way for that to happen

Because there is no legal process for them to become US citizens. 

Those who had any basis to do so have largely already done so. It's not like just anyone can enter the US without inspection and then legalize themselves. If it were that simple there wouldn't be any illegals living here. 

Because, as you said,  it's a legal process, and they are illegal, not undocumented.

Because they don't qualify to do so

Because not everybody can become a U.S. citizen.

What, you think it's just a question of walking over the border and filling out some forms, and then you're a citizen? It doesn't work that way.

They have to go back.