Exactly for the reason you say :

Market cap - what a company "owes" it's investors. Definitely on the liabilities side. The value of the company.

Cash - what a company definitely owns (and could, for instance, pay to it's investors)

So there is this measure of company's worth called "enterprise value" which is Market cap + debt - cash - assets. It sort-of is what you'd get if you acquired the company and immediately sold everything off. To put it another way : it's everything Microsoft has, other than the business it's involved in.

It can indicate that there are deals to be had. For instance if you pay $50 million for a company that has running accounts with $40 million on them, have you really paid $50 million for it ? You had control (through a company) over $50 million, and now you have control over $40 million and a company, so there is an argument to be made that you "only paid" $10 million.

So "enterprise value" is what you would really have to pay for a company, and for a lot of people considered a more useful metric than market cap.

What if you want to buy this company? His cash is your debt. The more he has, the more you need to borrow. The actual you get is the selling price minus the cash you get from this buying.