I wouldn't call them lazy in general. It's just all they can do in a reasonable manner.
Let's compare it to wc3. There, especially the beginning was easy, and that means a lot. WE was setting new standards in terms of available assets and also capabilities. Ten thousands of people opened World Editor, playing around and starting it as a kind of hobby, making low quality maps. Due to having fast first achievements many put more effort into it or created websites for exchange with a mass of users, leading to more buzz.
Of course there are limits in the engine and in new asset creation, as even when having many people on it, only a low percentage will succeed in making high quality assets.
As explained somewhere else, DotA 2 currently suffers from the same problem as sc2. For many reasons it's beginner unfriendly. If the masses of ordinary people passionately playing it don't learn and grow with it and create increasingly better content, who will? How many people who already are professionals will pick up DotA 2 modding? Not many. As they most often have the choice of either doing their thing in DotA 2, or doing it elsewhere as their job, earning money. In the last ~20 years we have seen it over and over again that great modding grows out of the people involved with the game being noobs at start but having spare time at their hands.
Unless you can earn a huge pile of money for DotA 2 maps, that won't change.
Over all this hovers the unbearable slow development speed of Valve though. Let's hope for cool (not too slow) updates.