There are more UV photons per unit area in artificial UV versus solar. UV kills living things because it breaks apart DNA molecules (specifically, it creates thymine and cytosine dimers which prevent DNA transcription and replication and create mutations in the sequence), and the cells die if they sustain enough damage, and artificial UV is so concentrated that it does this much more quickly than the sun, since cells do have the ability to repair DNA damage. It’s like the difference between playing catch with your friend or having the whole class throw dodgeballs at you all at once.