My take on this is that most US firms have outsourced first tier tech support to non-native English speakers who have fairly useless scripts they have to run through. You're dealing with a human (sometimes), but it's about the equivalent of dealing with a robot. It's hard to remember to have empathy when whatever you say is met with a standard, often nonsensical readout of the next thing in their script. So I think we've trained people to expect a horrible first tier experience.
That said, I've done a lot of B2B enterprise software support and have found exactly the same thing as you. Initial emails or calls will come in and the tone is aggressive and impatient. I think this stems from the assumption that the response will be useless (until maybe it gets escalated 3 times to someone actually useful). But when you respond as a capable human who legitimately is trying to help them out (and not just pass them on to someone else), suddenly the tone totally changes and you have wonderful interactions. People are incredibly appreciative. Nobody is used to a support person actually solving their problem. Hell, they're not even used to someone replying to them at all most of the time. The bar is on the floor. So when you exceed that bar and actually help someone quickly and efficiently, they turn into super fans.