because some of them, at least, feel that, in their artistic judgement, believe it better expresses the music and the intent. A point of view which is rather hard to argue with, as music is, if anything, a live performing art where both the composer and the artist must come together to produce the performance.
I might point out that there are some performers who are absolute fanatics about following the composer's markings to the last dot. They have their place too; it's a matter of taste, and in my view, at least, there is a place for both.
Same sort of thing happens in other instrumental music and choral music as well, of course.
I might also point out, however, that -- in general -- it is better when beginning to work on a piece to see what, as exactly as possible, the composer had in mind. Then later, depending on your own ability, taste, and skill you can modify that.