I think there are a few issues. The first is funding. If you are an engineering manager and trying to ship product, you don't want a 25% faster Ruby VM in 10 years, you want someone to benchmark and optimize existing code in existing products 25% today. I agree that it's a good pay off, cost versus benefit...however the benefit is spread around hundreds and thousands of companies. Google gains marketshare for chrome when V8 gets faster, there's very few companies that get significant advantage explicitly from making Ruby faster for everyone. If you're paying to make Ruby faster, it's also getting faster for your competitors too (if they use Ruby).
There are a bunch of core contributors that are very active. Several of their companies allow them to spend time contributing (most of them are from Japan). Another example is Aaron, he is on Ruby core and his company allows him some time to contribute to open source, so this is sponsorship in a way. I while a company stands to benefit from sponsoring a full time developer, they benefit just as much if someone else sponsors a full time developer. Right now there's not enough companies with either a business incentive, altruism, or interest. Perhaps there are companies out there interested in sponsoring full time devs who just don't know how (and to your point cannot find them) but I think that would be the minority case.
Oracle seems to spend some money on java and JRuby which is pretty fast and will get even faster the coming years. Not sure how much if any they are directly funding JRuby but they are funding the JVM which is used by JRuby.
There are a bunch of core contributors that are very active. Several of their companies allow them to spend time contributing (most of them are from Japan). Another example is Aaron, he is on Ruby core and his company allows him some time to contribute to open source, so this is sponsorship in a way. I while a company stands to benefit from sponsoring a full time developer, they benefit just as much if someone else sponsors a full time developer. Right now there's not enough companies with either a business incentive, altruism, or interest. Perhaps there are companies out there interested in sponsoring full time devs who just don't know how (and to your point cannot find them) but I think that would be the minority case.