1. You have to have a control group, so half of test subjects would be exposed to the virus without having received the vaccine.
2. The highest-priority recipients for a vaccine are members of high-risk groups; it'd be pointless to run a challenge trial full of low-risk individuals.
So a challenge trial wouldn't just mean giving the virus to a few thousand vaccinated macho 20-year-olds. It'd mean giving the virus to unvaccinated 80-year-old care home residents.
This is binary all-or-nothing thinking. It considers 90% the same as 0%, since both are not 100%.
Testing on healthy young people you learn how a normal immune system reacts to a vaccine candidate. High risk groups mostly have similar immune systems.
Even if somehow you could only vaccinate everyone under 60, that would do enormous good stopping the spread.
Thinking is not just "binary or not"; there are degrees. If exposing that group of people would be 90% pointless, it's not much less out of the question than if it were 100% pointless.
You're right about the principle, of course. I just think it would be "10% pointless", and thus well worth doing.