This is normal business. Many companies will not pay before the get the first notice as it reduces the risk to pay bills for POs which were never fullfilled. Also, many businesses give you a discount when you pay on time: https://de.m.wikipedia.org/wiki/Skonto (german) This practice is more popular in Europe because trust between companies is high (also because its harder to establish a business and you need assets as reserve).

I was talking about large, established suppliers dealing in large volume, not you, unless you have a special AR department with multiple employees. what these established suppliers love is sales, growth, and especially reorders. obviously they need to be paid but establishing repeat channels is the most important for them.

At the time, I had the perspective you just stated, and I thought large suppliers had the same perspective. they don't. (as the other reply to you stated.)