At the risk of being slightly offensive, college hires are high on energy and enthusiasm but low on ability to produce reliable code (on average). They can produce a lot of code, but they tend to have blind spots when spotting failure scenarios, resulting in "gotcha!" outages or bugs. Its not a big deal if they have a more experienced developer reviewing their commits, but if you're iterating fast on something that's going to be a flagship product, it becomes less tolerable.