Why don't the dwarves join Bilbo in exploring the tunnel in The Hobbit?

The classic fantasy novel The Hobbit, or There and Back Again by J. R. R. Tolkien tells the story of a journey to the Lonely Mountain by Thorin Oakenshield and his band of dwarves to recover their lost kingdom and its vast treasure from Smaug the horrifying dragon under the Lonely Mountain. In chapter 1, the company hires Bilbo as the expedition's burglar at the instigation of Gandalf the Wizard.
The tunnel the question refers to is a secret door in the side of the mountain. Thorin alludes to Bilbo's part in their quest when he says:

Aren't you the burglar? And isn't sitting on the doorstep your job, not to speak of getting inside the door?

In other words, the dwarves hire Bilbo to penetrate and enter the secret passage in the mountain once they have found it. When the dwarves and Bilbo finally locate the entrance and manage to get it open in chapter 12, "Insider Information," Thorin clarifies Bilbo's task:

Now is the time for our esteemed Mr. Baggins . . . to perform the service for which he was included in our Company.

Bilbo asks if any of the dwarves will go with him, but no one volunteers except Balin, who goes a short way in as a lookout. Tolkien explains that the reason the dwarves do not go in with Bilbo is that "dwarves are not heroes, but calculating folk with a great idea of the value of money." They hire Bilbo and intend to pay him well, because they are not willing to go in themselves—although they do attempt to rescue him when he gets into too much trouble.