Two reasons.
One, the Sun is brighter from your perspective at noon than it is in the morning or evening. That’s because the Sun is at a larger angle from the ground, and it’s the same reason midday is hotter than the morning.
And two, that change in angle also means sunlight travels through more of the air. Since air absorbs UV to some extent, longer paths through the air result in more UV being absorbed, and thus less of it hitting your skin. (This is the same reason why you get sunburned much more easily at high altitude.)
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