Why is still allowed in many states for minors to marry with parental, guardian, or judicial permission?

The definition of a child is not a natural or absolute thing. Where we draw the line in terms of age has varied across different times and places.
Currently, here in Canada, marriage is legal as of age 16. If a person is old enough to drive a car, a potentially deadly vehicle, at that age, I can't think of a strong reason not to allow them to marry.
Conversely, if we want to limit marriage to people who are fully mature, my understanding is that scientists now say that the brain is immature until about age 25. I am honestly concerned when I hear of couples in their early 20s marrying, but I wouldn't want to make it illegal.

The reality is that the age of maturity is not universally the same. As one post states, scientists say most brains are not mature until age 25. Another post brings back an argument from the military draft days of the 60's -- you can die for your country at 18 -- which was used to reduce voting age from 21 to 18. Now there is no real military draft, but you can vote at 18.

I would not recommend marriage under 18. I would not recommend marriage in the early 20's. Otoh, I have a niece who got married at 16 with her parents' permission. I thought it was a tragic mistake and wouldn't last. She is in her 50's now, married to the same guy, built a great life for them and their two children. It was her route out of what was a dysfunctional family; she chose well, and her parents' permission may have been the only gift they ever gave to any of their children, imho. Hard-fast, one-size fits all, laws cannot anticipate every situation. Having opt out provisions for the exceptions makes sense.

Those ages seem to change. When my wife and I married in 1967 Kansas said the guy had to be 21 and the girl 18 to get married without parental consent. We were backwards
A couple of years agoy grandson applied for the marriage license on line. Neither one was 21 and no blood test.

I believe in protecting our children. I fully support those laws.

Until 2021 it was 14 to get married here with parental consent.

Age of consent is 16 in general.

Then they changed it to 18 to get married without parental consent, 16 with.

But the age of consent is still 16.

Because we have no progressive values here, just traditional ones. 🙃

Because children can’t support themselves so they need mom n dads ok when they are under 18 so mom n dad can keep feeding them. Underage marriage isn’t progressive it’s just ignorance to think an underage person can financially support themselves.

You know what's worse? In states that allow child marriage, they say children aren't old enough to understand divorce papers. Meaning that once a child is in a marriage, they are not legally permitted to escape it.

Three letters....GOP and the evangelical church. These people literally believe if they are old enough to bleed they are old enough to breed.

Worlds run by perverts..

A lot of it has to do with thinking their child is marrying better off, it's very common around military bases for poor people to marry their teenage daughters to service men, Elvis Pressley is a prime example.