The word effective is being used in different ways
Is a helmet mechanically effective protecting the head. Yes it is. There are testing standards. I can put on a helmet and hit it with a stick and not even feel it where that same force on skull would draw blood.
UCI requires helmets. The NFL and NHL require helmets. In my city under 18 you must wear a helmet.
Then there is statistically effective. Is there statistical proof that the use of helmets actually reduced head injuries. Based on the amount of data it is likely to be inconclusive. No one is going to commission a controlled study of people to ride with and without helmets. They know darn well a helmet protects and they are not going to put people at risk - that is lawsuit waiting to happen. Even with review of hospital records if you have an equal portion of traumatic head injuries you cannot conclude helmets were not effective. There is good chance the people with helmets were going faster.
I don't need statistics or a slide rule to know a helmet is going protect my head.
If you need statistics to decide if a helmet is effective then be prepared to become a statistic.
Risk / convenience management
With anything you take risk. Driving to store in your car you take a risk. You mitigate risk by wearing a seat belt. In the early days a lot of people did not wear seat belts (and even some don't today). One of the excuses was - not yet proven they actually save lives.
There is actual risk and perceived risk and some people are risk averse and some are risk tolerant.
The OP asserts most vector don't include the head so is it effective? The problem with that logic is a force to the head becomes serious or even life threatening with a relatively small amount of force. If you land on a rock hard enough to break an arm that same force on an unprotected skull is easily life threatening.
There also is this vector of hit by a car.
As your speed goes up so does your chance of fall and severity impact. At very low speed and nothing out of your control (like a car) then the risk is manageable. The risk of a severe brain injury is very very low even without a helmet. A helmet still does provide protection.
In the winter when it is raining or snowing I wear a rock climbing helmet with a climbing jacket that will go over the top. A friend of mine said but it is not as safe as a certified bike helmet and my response was the helmet I am wearing is a lot safer than the helmet I am not wearing.
The NBA does not require helmets. It would alter play and there is not enough speed for a likely severe skull energy. And they are pros that control their falls.
If I am going to ride less than 10MPH and no cars then no helmet is a fair risk / convenience management. Yes in Holland with segregated cars going to market at low speed is fair risk / convenience management. The OP is new rider Nepal.
A buddy of mine bother's died playing basketball. They set up one of those mini tramps and were doing dunks. He hung on the rim swung forward lost his grip and landed on his head and boom dead.
I know a guy that hopped his front wheel doing less than 10 mph and lost the front wheel. He went out the front without a helmet and spent 6 weeks in the hospital. He wears a helmet now.
I was practicing low speed bunny hops and went out the back with a helmet on. I still passed out for while and had a terrible headache. That would have been a trip to the hospital and potentially life threatening without a helmet.
Look for reasons to wear a helmet not for excuses not to wear a helmet.
Buy a good helmet. A $100+ helmet rides a lot nicer and has better ventilation. Ride with your helmet firmly strapped.