JesusIsGodAlmighty it was probably done like that because it was the easier solution from a design standpoint. You'll notice it's actually always aligned to the left. On desktop it's simply aligned to the left of a smaller box that's on the right.
We might also need to take into account the growing width of mobile devices. On some "older" mobile devices I expect the current design looks pretty large.
If you would prefer to have it not occupy the full width on mobile, you could use some CSS to give it a fixed/max width, and push the new smaller box to the right. It would still be left-aligned, but it would be aligned with a point closer to the middle of the screen.
Also, while it might not seem obvious, the scrubber is actually full width. You can put your finger down on the right side and move it up or down just like you can do on the left.