Because if they didn’t they wouldn’t be very useful restraining people in a crash.
If you mean to ask how they do it, there is a mechanism that spins as the belt extends, flinging out an arm that catches on a toothed housing. This locks it in place as long as tension is applied.
Such a mechanism allows you to pull the belt out to put it on, but also stops you from going through the windshield in a collision.
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