Why? Because he's a billionaire and wanted to. It's charming when he does it and it makes news. If you or I tried to build a clock in a mountain, we'd be just some weirdos with shovels.
Anyway, yes, the clock. It's The Clock of the Long Now and it's designed to run for 10,000 years. At least.
This Clock is the first of many millennial Clocks the designers hope will be built around the world and throughout time. There is a second site for another Clock already purchased at the top of a mountain in eastern Nevada, a site surrounded by a very large grove of 5,000-year-old bristlecone pines. Appropriately, bristlecone pines are among the longest-lived organisms on the planet. The designers of the Clock in Texas expect its chimes will keep ringing twice as long as the oldest 5 millennia-old bristlecone pine. Ten thousand years is about the age of civilization, so a 10K-year Clock would measure out a future of civilization equal to its past. That assumes we are in the middle of whatever journey we are on - an implicit statement of optimism.
It's being built on land Bezos owns and he's involved in the design and construction. It's going to be 200 feet tall.
AND I WANT TO GO SEE THIS CLOCK.