"Faith" is a big word, but for many/most haredi people "community" is the more operative term. Leaving a sect means becoming an outsider. All their family and friend relationships are insider-based. Regardless of the severity of luteral shunning practices, the nature or haredi lifestyle makes these aspects severe regardless. There's a lot of variance between communities though.

In any case, people do leave. Often, it's relatively early in life. This is why haredi communities are so leery of military service, college lifestyles and such.

People also join. A lot of Chabad adjacent communities are largely converts and 2nd generation members.

I would also emphasize that lifestyle, community, belonging, mission and such tend to play as big a role as belief in convincing converts.

Judaism doesn't always emphasize belief in its definition of "faith." At least, far less than Christianity and Islam. This a philosophical tradition going way back. Faithfulness and even spirituality are often seen as concerning practice (ritual, but also moral) moreso than concerning faith.

Belief follows practice, and if it doesn't... at least you have practice. The Christian Brothers pinched this line from an old rabbi.