At the equator of the earth, the surface of the earth is rotating eastward at about 1040 mph. Earth orbits the Sun at about 67,000 mph, and the Sun orbits the Milky Way at about 486,000 mph. So what is the speed of an object moving 10mph due east at the equator, compared to an object in outer space?
You and a friend riding a train at 40 mph. Your friend asks you to toss him some object.. ah, a cell phone. Don't drop it, lest it break! You underhand toss it at about 3 mph, he smoothely catches it. An observer standing still in the subway terminal, waiting to catch their train, sees you phone travelling at 40+3 mph. As Jeff Reid pointed out, you could also add up the speed of the Earth's rotation, Earth's orbit around the sun, the velocity of the Mily Way, etc, and reach some ridiculously high number relavent to some absolutely stationary observer (which quite posibly can't be known to exist, since everything is relative). Speeds are relative, speeds cancel out. This is in accordance with a Newtonian universe even.
It seems as if these question based answers arent helping. Motion is relative because think of a moving car. You are driving 100 mph (relative to an object at rest) on the highway, and a cop (at rest) clocks you going 100. Now if that same cop is traveling next to you at say, 80 mph, the radar only clocks you as going 20 mph. (100 - 80 = 20). So therefore we say motion is relative. Answer your question?
By definition, a "velocity" is a change in position between two points, with time. If you're asking why there is no absolute reference frame, that's a little more involved...