Because the human brain is bad at probability and susceptible to hacking.

And on the other like-a-fox hand, the military does have a strong incentive to identify individuals susceptible to developing gambling addictions.

It's cold, but putting them in close proximity to available gambling isn't the worst test...

It's the government, you may as well have another set of people observing the test givers as well.

Because slot machines are synthetic dopamine generators and humans have thousands of years of evolution tweaking us to favor dopamine-generating activities.

“Addiction By Design” by Natasha Dow Schüll is a good book to answer this question.

I'm reminded of an anecdote I once heard, which I can't readily find atm, that in the days of yore when Chicago dominated the pinball industry, the same complex also dominated the slot machine industry. This business was later purchased by Bally's and moved to Las Vegas, where it appears the book picks up. Along with the business came the statistician whose job it was to make slot machines addictive. He later came out of retirement to work for Tinder. Don't really know if that last part is true, maybe someone here does.

That's like asking "Why doesn't it hurt when a tick latches onto your leg and starts sucking blood?"

They've been designed (evolved) for a purpose.

Pulling a lever/pushing a button should get boring. Slots are designed to not be boring while they bleed you dry.

Because they're designed to be addictive.