I think if you view marriage as "just sitting around," this is not the type of romantic relationship that describes what typical founders likely seek out.
To answer your point more directly, one difference is that a marriage tends to produce children in a way that platonic friendships tend not do. My sense is that equal division of significant income and equity post-marriage will help, not hurt, children.
To answer your point a different way, the default marital property settings tend to work well for most people. Of course you can change those defaults to some extent with prenups, postnups, trusts, and so on. If you're still dissatisfied with your ability to alter those defaults, well, nobody's forcing you at gunpoint to marry someone you like to live with, have sex with, and even raise kids with.
To answer your point more directly, one difference is that a marriage tends to produce children in a way that platonic friendships tend not do. My sense is that equal division of significant income and equity post-marriage will help, not hurt, children.
To answer your point a different way, the default marital property settings tend to work well for most people. Of course you can change those defaults to some extent with prenups, postnups, trusts, and so on. If you're still dissatisfied with your ability to alter those defaults, well, nobody's forcing you at gunpoint to marry someone you like to live with, have sex with, and even raise kids with.