Quick answer: Your cat sleeps on your chest because it's warm, your heartbeat and breathing are soothing, and sleeping on you is one of the strongest expressions of trust a cat can show.
Cats have a higher body temperature than humans and actively seek warm spots. Your chest is warm, rises and falls rhythmically, and is elevated off the floor — combining three things cats prefer in a sleep spot. It's physically one of the most appealing spots in the house for a cat.
Cats only sleep deeply where they feel completely safe. Sleeping on you — especially at face or chest level — means your cat trusts you fully. In the wild, cats are vulnerable when asleep, so choosing to sleep on you is a significant behavioral expression of security.
The rhythm of a human heartbeat is thought to be reminiscent of sounds kittens hear near their mother. Many cats are drawn to the sound and gentle vibration of a resting heartbeat, particularly when it's slow and calm during sleep.
It's a genuine expression of trust and comfort. If you find it disruptive, redirect gradually to a heated cat bed placed beside you rather than pushing your cat away.