Star formation takes a pretty long time on human-historical timescales. It also tends to happen inside of nebulae, which can make it hard to see. I think that most main-sequence stars also take a while (thousands of years, at least) to blow up into giants when they start to run short on fuel.

There are variable stars that go through really weird, massive changes in brightness, and for all I know, some of them might have become persistently brighter after being inconspicuously dim for a long time. But I don't know of this happening to one of the small subset of stars that are bright enough to be naked-eye visible from Earth. There are several that become very dim and very bright periodically.