The three bad reasons

The first is that you are good at the subject now. Being good at school physics at fifteen predicts very little about wanting to do engineering at twenty-five, because school physics and engineering share a name and almost nothing else. The second is that a relative did it and it worked out. One person is not evidence, and the labour market they entered no longer exists in the same shape.

The third and most common is keeping options open for its own sake. Taken far enough it produces someone who spends two years in a stream they dislike, performs adequately, and arrives at the same decision with less energy and worse marks. Optionality is worth something. It is not worth everything, and it is not free.