Slightly related: One thing I've learnt in Physics is that there are still a huge number of practical and fundamental problems to be solved. You might expect everything easy will already be done by now but that's just not true. Hence we still manage to write so many papers! In fact many Physicists have too many papers they want to write and not enough time to write them. This was a huge surprise for me coming in to my PhD. I had naively assumed that physicists were all scrounging for any morsel of content to publish!!

Of course it depends on the field. It's a requirement here in the UK to publish a paper to get your PhD (at least I've been led to believe that), and many particle physics students really struggle to manage to fulfill this. On the other hand gravitational wave detectors have a huge amount of unexplored science as there are so many things that we still do not fully understand about the detectors themselves.