In tech, there are many accredited (potential) investors who have product expertise but no finance experience. This "slack capacity" of capital is what Carta wants to profit from. (If you want a public VC fund, there's Forge. Maybe Carta wants to go here some day, too).
Why do tech employees join companies that doll out significantly better equity to investors? (In some cases, those investors even get dividends on their convertible debt). Why are there still startups who don't even have 83(b) elections yet, and why aren't all start-ups doing bonus-to-cover early exercise? Why are employees totally cool with dilution and a 10% employee pool?
Because employees earn just enough to compete with each other, but not with the house. This is Carta's business opportunity.