Why is paying for backlinks not considered a black hat strategy?

As a medium sized company in a large city, I feel somewhat handicapped with SEO. I have reached out to 20-30 companies trying to organically produce backlinks without a single response.

I think if we spent a lot more time, we could probably produce some results but they would be few and far in between with DAs between 20-40.

It just doesn’t seem right to me paying for backlinks in the way that I understand it - a third party agency acting as a middle man for two companies - should reflect well on a website.

I’m probably going to have to move forward with paying an agency, but my question is why is this not a frowned upon practice?