Yes, there has been a standard format for invoices for decades, but it was only ever used if both companies were using a ERM system (and as you say, large enough purchasers could force their suppliers to). We have to deal with small business who don't use the standard format, which is the vast majority of them.

Please go ahead and try parsing non-standard invoices without an LLM. I spent 20+ years on and off dealing with this problem. It's not as simple as it looks. And then LLMs came along and made it simple.