I think they’re hoping that when they break up (as they inevitably will) it will be a smaller legal mess if they’re not married and bound by all the state laws that affect property division, alimony, and other considerations. Probably their lawyers and accountants told them to stay unmarried, whether that was good advice or not.
Well, that's it. Marriage is now a government-administered social program, and one has to carefully evaluate its regulatory requirements before deciding to register and disclose personal information. ;)