Why is hydrogen not burned for cars instead of hydrogen fuel cells?

They can, actually there are hundreds of DIY'ers that have converted their engines over to run on hydrogen. But, they have to get their hydrogen from home. Because federal regulations do not allow for public stations for hydrogen. A spark of any sort, even a cold spark can ignite hydrogen. That is why blimps are not as popular as they would have been had the Hindenburg hadn't exploded.