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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This is actually really difficult. The wall of the tunnel basically forms an archway that supports the weight of the rock and soil above it, and holds back the pressure pushing in from the sides. If the tunnel is dug too deep or the wall material isn’t strong enough, the walls will shatter under the strain and the tunnel may collapse.
There are lots of techniques for improving the strength of the tunnel walls. Sometimes huge steel bolts are drilled into the walls to attach the tunnel wall rock to rock deeper inside the mountain. Sometimes grout is injected into the rock, or concrete is sprayed onto the walls to glue everything together. For really weak material like sand or soil, sometimes a whole reinforced concrete tube is built to support the load.
Tunnels do occasionally [collapse](https://en.wikipedia.org/wiki/Category:Tunnel_disasters), but not as often as mines do. Mines face many of the same engineering challenges as tunnels, but with a mine the goal is to remove as much rock as possible, whereas with a tunnel you save money by removing as little as you can.
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