Attacking reconnaissance satellites is fairly cheap for anyone with even a sub-orbital rocket and can provide a huge amount of military value. If you can make it to geostationary orbit you could cause a lot of havoc, too. It could also trigger Kessler Syndrome which is why we need to take it seriously.

However, my headcanon is that the Space Force was created simply to get the SGC more funding. They even made a TV show about it.