HL7 standards are developed on a consensus basis by providers, payers, vendors, governments, and academics. The established vendors certainly help, but I deal with their HL7 representatives on a daily basis and I can clearly see that they're working in good faith. The main reason there is flexibility (and convolutions) in the standards is that healthcare is highly complex. Integration projects are thus labor intensive; software is usually a relatively minor slice of the total cost.

If you have a specific suggestion for simplifying or otherwise improving any HL7 standard then please submit a formal change request. The process is very open and accessible. I've done it many times myself.