Not OP but many many reasons. If you have the money and you prefer comfort (and/or have kids along) taking a taxi/uber/etc. is much more preferable than dragging several (probably heavy) luggages up and down platforms (elevators may or may not work), walking long paths, etc. especially when you consider the alternative is simply putting luggages in a trunk, sit down and relax, and most likely get there faster. And all of this is before we even talk about any safety issues with BART.