I had the exact same experience! I reached out to a bunch of providers when I started and not one responded. I think they are missing out as they don't understand how beneficial the relationship is. I had better luck meeting providers in person and have referred to them when full. I doubt it is reciprocated as they seem to be intimidated by the competition regardless.

I also think part of it is just the nature of the job. We are sometimes inundated with emails (with already a difficult work/life balance) and I am not the best at responding promptly (with plans to get to it later) unless I have a spot to fill.