Ask yourself this: How many consumers out there own video cards that fully support DirectX 9? Hell, I know a few Compaq owners that are still running TNT2 cards. Why would you, as a game developer, fully support a standard that most users couldn't possibly take advantage of? The majority of people are still running DirectX 7 or 8 cards. That is why the engines are not fully supported. They have to be backwards compatible with all the old crap that's still out there. Writing software to fully support each version of DX is obviously time-consuming and therefore costs money. Something gaming companies don't always have a lot of.
Most games today would not run on a TNT2 anyway so that point is moot. All Geforces and up support hardwired T&L and yet...so few games utilize it.
The same is true of SSE. Most games today wouldn't run very well on something like a Pentium 2. Most people have a P3 or up or an AthlonXP. Why, then, are so few games optimized for it?
"We are Microsoft, resistance is futile." - Bill Gates, 2015.