A lot of things seem weird when you don't think them through.
That is quite a sense of entitlement, to ask to be able to use the network with a random car that does not have the proper internal equipment.
Tesla made a better connector, early on. It offered to other car manufacturers the opportunity to join together in funding the network. Other car manufacturers declined, and continue to decline, as is their right.
Why does the manufacturer need to be involved? Well there is equipment required inside the car, which is too big to be simply an adapter.
This equipment costs some money, but as I understand it, it makes the car more energy efficient. This may be part of why other brands have poor relative efficiency, dragging around more battery weight for less range. Tesla has to pay for this better equipment, which gets added on to the price of the car, but then you get the money back in better driving, better energy usage, and a better charging network, as well as the ability to charge at all most other networks. Other manufacturers want their cars to appear cheaper, so they cut costs, and users pay the price in daily use instead of in the initial price tag.
Why would you buy a car that costs $10,000 more? Most people don't go the extra step and think through that there might be some actual reasons to do so, and their investment of that extra money might be wise. The thinking just stops at the number.
I am glad that random people who think like this can't just bring their own super cheap DIY / bought on Amazon / Ebay / Aliexpress sketch Frankenstein AC/DC converters and adapters, which may not have proper protection mechanisms, and plug in. The network has been very reliable and I would not want to see that change.
When a car manufacturer is involved, there are at least safety measures, proper design, and recourse and a feedback look for improvements if anything goes wrong. I mean, this is basic stuff.