What business does the state have licensing ANYTHING, except to generate income for itself?
Licensing at times is done for safety (establish that you can pass a test and be prohibited from driving, practicing medicine, etc.).
Marriage licenses can protect against people who are closely related marrying (which could lead to health problems, but not necessarily), blood tests are administered (to inform potential spouses of possible STDs?), they can also protect against bigamy.
We have to renew our drivers' licenses (to make sure that the address is current, update the photo every 8 years or so, and collect more revenue), inspection tags verify that our car is safe to operate other drivers (and collect revenue), we DON'T have to renew a marriage license, not even when we move.