Consider the opposite. The company has a market cap of $100bn and $10bn in cash. If you buy the company out for $100bn, you automatically also get $10bn cashbac

Consider the opposite. The company has a market cap of $100bn and $10bn in cash. If you buy the company out for $100bn, you automatically also get $10bn cashback. So it's more like it is worth 90bn, plus has some cash attached - but that bit is a wash. Debt is negative cash, so instead of subtracting it from market cap, you add it in.