Bill Gates is also applying his money where he thinks it will have the most impact - their foundation often invests in new technologies and startups whose products will have the greatest impact per dollar. If Gates and Musk are both working to maximize impact their difference is who they focus on. Gates primarily focuses on the most impoverished on the planet, while Musk focuses on the first world (and indirectly the most impoverished due to the effects on climate change).
Saying that the Gates Foundation does not reinvest into new ideas that maximize impact does a great disservice to what is one of most effective and effectiveness-oriented philanthropic organizations on the planet.