Similarly in technology, there is much to do, but tinkering with hardware, craftsmanship; the things that worked in industrial labs in the old days: they're not done any more. Even making some simple piece of junk aluminum part, people waste time fooling around with solid designer and FEA instead of just handing a piece of graph paper to a former Navy machinist. There are entire books written in the post WW-2 era about fast development when people actually used to develop things quickly. Nobody does it. Well, the Russians and Chinese do, but they also develop big technology a lot faster than we do in the west.
I have no idea what's wrong with machine learning research; probably gratuitous abuse of grad students, people jumping after fashion, and lately brain drain.
I'd argue that the more we learn the harder it is to find major breakthroughs. Looking at Math and Physics as a model, I think this is clear, that major breakthroughs become more sparse as time passes. Luckily we have more eyes looking at things now, which helps a lot.