It's not about complexity, it's about convenience. The simplest bid on eBay is... the highest price you'll pay for it.

However, most people didn't think of it that way or want to do that (believe me I worked there for years), and they didn't like the chance to lose it anyway after waiting (snipers and bid shilling got ridiculous).

Fixed price is easier and you get it quicker. It may be less complex but if the price looks reasonable it's definitely more convenient.