Why can't people inside cults see what outsiders see?

It is low self worth.

They think joining a cult makes them better.

As a general rule, cults demonize critical thinking. The people who are drawn to them have deep needs that the cult fulfills. To think critically is to risk being kicked out. Consequently, people who cannot or are unwilling to think critically are drawn to charismatic cult leaders and once there, critical thinking is frowned upon.

Because when a person has a deficit in IQ they rely on others who they deem as intelligent or rules that are outlined for them thus making it easier for them to be brainwashed. They lack the intellect to do their own research and make up their own minds for that would require the ability to critically think.

Because Catholics are brainwashed.