I know there can be cave-ins where a slice of rock is sheared off of the inside of the tunnel, but as far as I know tunnels don’t just outright crumble and collapse. Why not?
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The weight of the stone above the tunnel is distributed to the sides, similarly to how it works with bridges. However the entire weight is not carried by the concrete walls of the tunnel, but also by the rock around them. If the tunnel goes through sand instead of solid rock, the tunnel walls would need to do most of the heavy lifting.
The round shape of the tunnel can help distribute the load evenly, but is not necessary in solid rock with enough stability. Natural caves come in all shapes and sizes if the type of rock allows it.
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