I think it's also kinda complicated. The machines don't do it well by themselves, so to be effective, especially if you are purchasing a lot of hashpower, you pretty much have to set up a pool.
Solo mining would be a lot simpler and more viable if someone capable could optimize GBT a little better so that it's viable to use on networked miners built on embedded platforms like the KnC miners.
You can solo mine without a pool easily using BFGMiner's capability to talk GBT straight to bitcoind/bitcoin-qt, but it is too heavy-weight to run on a Raspberry Pi or a Beagle Bone Black as it is now.
If you're using USB-based ASIC miners, like BFL, you can use BFGMiner right now to solo mine without a pool. If the miner has to run on something less powerful than a traditional PC though, odds are that it won't have the horsepower to handle GBT mining as it exists now.