It's not logical, is it? I think this is one of those things that you just have to accept as an idiomatic oddity of the language.
I would add that it is far more common to say 'half an hour' or '30 minutes'. In fact, I really can't imagine a situation in which people would choose to use 'point five hours' (or 'zero point five hours') instead.
On the other hand, I believe there are style manuals that DO recommend using the singular form of the noun with a decimal that is less than 1. However, in everyday usage, I believe the vast majority of native speakers will use the plural form for for decimals that are less than one. I'd say the only exception might be .1 (point one).
Here is one more usage example I turned up in COCA:
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