On the contrary, you can have tennis shoes which don't support you properly and end up with damaged ankles, knees and ligament damage. I get what you are saying about riding hats but we have tested CO hats when Ds was thrown off and hit his head on a rock and got up and walked away, admittedly with a deep dent in the front of his hat.
Whilst soft tissue damage in ankles and knees is an absolute bugger (I would know!), it's not remotely comparable to a life changing head injury (and, indeed, the chances of developing significant soft tissue degeneration within the lifespan of one child's set of tennis shoes is fairly low compared to the risk of head injury which only requires one incident).
I don't think you do understand my point as you seem to think that I'm insulting CO hats - I'm really not, I have several myself. However, not all hats, even by one manufacturer, are created equal, nor will a good hat necessarily protect every individual well. Human heads are vastly different shapes, and hats have to be selected to fit that. I have a CO Ayr8 which fits nicely. I cannot wear a CO 4Star, but my ProII is perfect. These are all the same manufacturer, but span a significant price range, and fit differently.
It's all very well to say "I can get a hat for £50", but that's meaningless if 50% of other people would have to spend £100. That is the point - if you want to buy trainers for entry level tennis training, you go to your local sports / shoe shop and can try a dozen makes at an entry level pricing. If you want to ride horses and go to buy a hat, you have one or two entry level price hats. If they don't fit, and you don't want to risk your kid having a serious head injury, you have to pay a lot more. That's the very definition of "inaccessible" on the grounds of finance.
Of course, this is tangential to the real costs of riding, which come primarily (at RS level) from the massive expenses of insurance, land (in the SE at least) and the labour intensive nature of keeping horses.