Actually, I don't think astronomers are very surprised. Even when I studied astronomy 40 years ago, it was understood planets were most likely a normal by-product of the star formation process, and that our own solar system was probably quite ordinary. There was no data, of course, so we couldn't say so for sure, but it seemed to be, and turned out to be, more or less correct.

However, when I first starting reading about astronomy as a kid, planets were thought to be extremely rare, caused by tidal effects when two stars suffered a near-collision; an extremely rare event, which meant planets were extremely rare in the universe..

The nebular hypothesis of planetary formation had been proposed had been proposed, but rejected because there was no known mechanism for the angular momentum of a spinning circumstellar disk to be dissipated. But by the time I was in college, that objection had been overcome (the angular momentum is transferred magnetically to the surrounding gas), and the nebular hypothesis seemed to be the right explanation for planetary formation.

This meant planets were probably very common, but we had no way of knowing for sure.