1. They can "pay" people with "free" pieces of paper that effectively cost nothing from a cash standpoint
2. It helps keep staff onboard by slapping golden handcuffs on
3. In the event that these paper options turn into something with actual value that only happens if the founders and investors make a ton of money first, so at that point they don't really care what the options "cost". It's like writing a paycheck that can only be cashed if the founders/investors get rich. A great deal for them, not so great for you.
Net net all these things benefit the founders/investors and not the person receiving the options. In nearly all cases people are getting options as part of core comp because the company can't afford to pay out all that cash. It's important potential employees understand that when agreeing to a base package that is heavily in options vs cold cash. Options should be treated as a bonus that may pay off but very likely won't, not base comp.