Like you said, only a third of our population has the chance to learn magic. Not even half of those reach levels beyond making a simple breakfast with magic or operating a quill. Why would the few magically excelling Unicorns do the work for the rest of us? From an economic standpoint we need to produce something in return. From a philosophical standpoint, we Pegasi, Earthponies and average Unicorns prefer to do things with methods that are within our means, e.g. with science and technology, rather than living idly under the patronage of the mighty magic-users.
Also magic might seem almighty at first, but you will find some restrictions: It’s impossible to create edible, nutritious food with magic alone. You can use magic for planting and harvesting, but that’s all. Another restriction is computation and the transfer of information. There are no spells to solve equations, to control processes like a machine could do, no magical constructs that would allow for individual communication like the Internet does. The Information Age’s greatest engineers were mostly Pegasi!
Why Pegasi, you might ask? Well, because Pegasi are the best mathematicians. Our brains have developed to work in 3 dimensions much better than the ground-dwellers. That’s why most of us are very familiar with geometry. Also because of living above the clouds nothing blocks our view to the stars. Pegasi were the founders of Astronomy (never ask us about Astrology – the ‘l’ stands for lie), which we needed for navigation, but later forced us to deal with numbers and equations and to invent the basics of modern Physics! This gave us a head-start into relativity and quantum-mechanics.
Similarly the Earthponies understood the need to research soil and life itself in order to revolutionize agriculture. Earthponies are the best geologists, they developed chemistry and searched for the origins of life. They discovered the secrets of cells and finally the DNA.
There are a lot of smart Unicorns in every field of science, but they are a big minority. Nobleponies can’t stand having to ask common ponies for tutoring and getting corrected all the time. A lot of young magical Unicorns would agree with your question: Why study nature, when you can bend it over your will? Despite a big interest in History and Geography they avoid Archeology and Geology, because they link it to dirty work. I have heard rumours, that quite a number of noble, young Unicorns research Economy, Psychology and political studies, thinking it will give back lost wealth and power to nobility, again. I won’t comment on that, however.