I'm not that deep into racing, but I'll guess that teams still want to be way under that weight so they can strategically place weight where they want it.

They may want it, but the technology only recently converged to the point where you can have an aerodynamically optimized bike under the limit weight. Before that there were special lightweight ones for the mountain stages and heavier but aerodynamically faster ones for the flatter stages. Most teams still got these two, plus of course another bike for time trial, because time trial stages are not mass start and the bikes and rider positions can be even faster and less safe there.

Some bikes are over the minimum weight, by a wide margin, and rode by the race leaders too.