They don't actually. Ad agencies have people who do this stuff for a living (generating what they feel will be the optimal ads). Agencies have been held back by these people in a way, because they are naturally very resistive to someone telling them that a computer might be able to do their job better than they can, even if the end result may not be as aesthetically pleasing to them.
In terms of automating the variations, that can be done ahead of time with photoshop scripting/etc, and then leaving it up to the adserver's optimization algorithms to serve the best-performing creative.
you're talking a couple generations behind what we're doing. there is a huge difference between plugging 10 variations into an ad server and using a genetic algorithm to quickly and efficiently find a local (or global) maxima in a search space of over 1 billion possibilities with constantly shifting preferences.
smartads is also a very different product, very focused on offer optimization, with a rigid system that doesn't expand well beyond the specific verticals they've created templates for. most players in this space have fairly limited optimization abilities, ie a/b testing, which does not scale up.