Why didn't congress just say no to appointing an unqualified anti-vax nutjob as health secretary?

Because the majority party that controls Congress is a mix of unqualified anti-vax nutjobs, and others who are willing to side with said nut jobs to further their agendas.

Because trump told them to appoint him

Funeral home lobbyists

Because Old Orange is scary

I think The Offspring put it best:

Shit is fucked up!