Short answer: only things like light waves moving at the universal speed c have the same speed in all frames, not objects like balls moving at slower speeds. L

Short answer: only things like light waves moving at the universal speed c have the same speed in all frames, not objects like balls moving at slower speeds. Longer answer. The principle of relativity doesn’t claim that particular objects have the same speed in different frames, it claims that the fundamental laws of nature take the same form in different frames. In the 1860s Maxwell wrote down a set of four equations governing ripples in the electromagnetic field (=light waves). They naively seem to only be exactly true in one frame of reference since they predict the light waves move at one particular speed c. Einstein made the leap to suppose that the Maxwell equations continue to take the same form in all frames of reference, so that each observer sees the ripples moving at the same speed c. This doesn’t apply to balls because the fundamental laws make no such prediction for the speed of a particular ball. (Obviously! You can hold the ball, or throw it.) Hence the ball being different speeds in different frames is just something about a particular ball — it doesn’t contain a grander implication about the laws of nature.