In general you want to have an easy to use sugar to bottle with. Having to mash grains just to get bottling sugar would be a lot of work, plus the gravity would be very low compared to what you need to bottle with, so you'd have add a lot of wort to the beer before you bottle.
DME is relatively low cost for small amounts and easy to keep on hand for bottling needs, starters, raising the gravity of your wort if it came out low, etc.
You can also bottle with table sugar, honey, corn syrup, etc. I've read corn syrup leaves the least sediment in the bottle after the carbonation process is done, so I've been using it recently.