They have the most cash reserves because they're (a) incredibly profitable due to product demand and monopolistic practices (i.e. patent suits); (b) stockpiling cash. It's that simple. There's plenty of things they could be improving at Apple or just investing in. They're hoarding instead. They're not alone: many of the greediest, richest, and shareholder-oriented companies are doing the same thing while stagnating.
It's not like having fewer people per project hasn't worked out for them in the one way that really matters for a company -- profit.