OEM mobos suck. Boards from this era of Optiplex are pretty standard apart from the PSU connector, but newer ones have motherboards that go all the way from one side of the chassis to the other, and mount the power button and front panel I/O on the mobo. I don't even know if they have internal USB headers.
They're not shitty. HP, Lenovo do the same shit - proprietary motherboards, limited feature sets, and minimal upgrade paths are typical of OEMs... It's also why a lot of people recommend building to anyone.