if the light bulb is screwed into the fixture why does it still burn out eventually?
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I’m assuming you’re talking about incandescent lightbulbs, not LEDs or florescents.
In an incandescent light bulb, a small piece of metal (the “filament”) is heated to very high temperatures by running electricity through it. To glow, the metal needs to be at a temperature of about 2000-3000 degrees Celsius (a bit cooler than the surface of the Sun, which is glowing [roughly the same color](https://en.wikipedia.org/wiki/Color_temperature) for the same reason). This is hot enough that even metal starts to evaporate at a reasonable rate, the same way that water left out in your kitchen evaporates slowly even well below its boiling point. While we choose a metal that evaporates as little as possible (typically tungsten), the evaporation isn’t zero, so small amounts of the filament evaporate away over time.
This process happens even faster if there’s small impurities in the “air” (actually some unreactive gas like argon) inside the bulb, which causes chemical reactions to eat away at the filament. As the tungsten bonds with other atoms, it forms compounds with much lower boiling points than tungsten metal, which evaporate much faster.
Eventually, so much of the metal has evaporated that part of the filament thins. That part becomes the hottest, which accelerates the evaporation further, until that segment of the filament finally evaporates away entirely and the filament breaks. This stops electricity from flowing, which stops the filament from heating up, which makes the light not work.
Everything that moves or gets hot etc will eventually wear out. So a light bulb gets turned on and off a lot. It gets hot, then cools down, then gets hot, then cools down etc. All this puts strain on them. And eventually they burn out.
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