Which gives most fantasy setting a very unrealistic/unbelievable feel. Humans have always used the tools at hand to improve their life, so why wouldn't they do that with magic?
It depends on the magic and how it works. You can take ten people off the street and teach them how to build a mill or a simple bridge in fairly short order, provided they don't have any grossly abnormal physical or mental defects. People built mills and bridges centuries before there was any such thing as a dicipline of engineering.
Magic, as it's usually portrayed,
isn't something 'just anyone' can do. Sure, there are some settings where it is and those settings tend to be the ones where magic is used much like we use technology. Take the Ethshar series by Lawrence Watt-Evans: almost anyone who wants to pay attention can learn certain types of magic, and they treat magic as another type of crafting.
Usually, though, magic is something only special people can do, or requires a great deal of time and training before it can be used - thus, there are not many practitioners. New knowledge and new techniques spread slowly. Also, wizards are traditionally jealous of their art. Science and progress depends on people who share what they know, and who share it relatively unselfishly. Wizards traditionally are exactly the opposite. They don't
want other wizards doing what they can do; they accumulate power for it's own sake. They also don't see why they should enrich the lives of those who didn't endure the work and danger of becoming a wizard themselves. You should know several professional people in your personal life that are exactly like that; they enjoy having power over others and will never give up something that makes them special.
Another reason is that magic in most stories is not science. Science is reproducable. If I invent a new type of plow, draw up that diagram and send it off to the blacksmith in a distant village then he can make more of them. Lots of times, you can't do that with magic. Many wizards spend most of their time constantly re-inventing the wheel.
Think of wizards as artists instead of scientists. You have a written peice of music. It has notes and bars and all the rest; it's like a formula. But each performance is different because the conductor - the wizard - is different. Working from the exact same 'plan', three different conductors will effectively produce three seperate peices of music. Compare the timeline of advances in music with advances in science; music has a significant advance about once a century. The big advance last century was Jazz, which lead to Rock and Roll.
