Explain Like I'm Five | Don't Panic!

Today I saw a self-portrait of Van Gogh, among other notable pieces including many by Rembrandt, in Amsterdam. Most of these artworks were not covered, I looked quite closely at many of them, and people are allowed within inches.

As a visitor I greatly appreciate the ability to see these works so closely but the pragmatist in me was a little horrified at how exposed they are. Mentally unbalanced people, those with political agendas, even fame seekers could so easily damage these beautiful works. There were museum attendants, of course, but they were some meters away and anyone could do irreparable harm to these irreplaceable paintings before the observers could react.