I’m no scientist, but I think it has something to do with actually turning the light off and on that’s actually stressful to most light bulbs. I mean check out the Centennial Light (wiki link). I know it’s an entirely different type of lightbulb, but they have run that light almost continuously since 1901, and it’s largely believed that continuous operation has kept it going for so long (though it has dimmed quite significantly).

I suspect that 20,000 hours operation is likely expected under continuous illumination, not ever turning the light off.