Schizophrenia isn't particularly subjective.
Sure it is. Read the DSM and see for yourself.
From the definition:
There is currently no physical or lab test that can absolutely diagnose schizophrenia - a psychiatrist usually comes to the diagnosis based on clinical symptoms. What physical testing can do is rule out a lot of other conditions (seizure disorders, metabolic disorders, thyroid disfunction, brain tumor, street drug use, etc) that sometimes have similar symptoms.
And further:
People diagnosed with schizophrenia usually experience a combination of positive and negative symptoms. These may include (but are not limited to) racing or uncontrollable thoughts, uncontrollable mannerisms, talking to yourself, paranoia, hallucinations or delusions, sensing that people are following or talking to you, insomnia.
So, what some might think is schizophrenic behavior might be an underlying physical ailment that goes undiagnosed, or a temporary condition that improves when something that is worrying and causing insomnia (like drug abuse, an imminent job loss or wondering where you'll get food for tomorrow) gets resolved.
No mental illness is objective - if it was it'd be a disease of the body. The brain is an organ too, and we have neurologists who repair brain diseases. It's a crucial distinction.