Because it doesn't work. Unless there's very few of said item available at a time undercutting by 1 copper means that i'd have to check my auctions constantly which i don't want to do. Try undercutting by 1c on a fast moving market and then see if any of your items sell. In order for that to have a good chance of working you also need to know when items sell or not. If it's a Tuesday and i post Potions of Prolonged power i'll probably not undercut the lowest because the price is at one of the lowest points during the week. In order to reliably sell those for good profit i need to know the price per week day and how it moves between that, the ~amount of items sold so that i can put them up at a reasonable price i.e. if i'm reasonably sure that 3k pots will sell in the next couple of hours i can put mine just below the price of item #2k to account for extra listings.
The above applies for more or less anything where the items aren't extremely scarce i.e. rare transmog items. On slower moving items i.e. obliterum undercutting by 1 copper will only fuck you over if you don't know how the prices move throughout the week(as slight as that is) as well as the current price trend for obliterum overall. Undercutting by 1 copper doesn't achieve that much. You get slightly more profit at the risk of your auctions not selling unless you know what you're doing. Surprisingly a lot of people aren't interested in price trends for items, price variance during the week, etc. so undercutting by 5% is a good start point that guarantees you that your items will sell if they're around the actual market price.