Why can't I add keyframes to dimensions I exported an .abc file from Marvelous Designer which doesn't show any keyframes of the animation in blender, but in the 3D view it animates.

However when I click the play button it shows the dimensions changing, indicating an animation is present, and I was wondering if there is a way to add keyframes as I am looking at exporting a USDz file with animation.
Dimensions are just a byproduct from actual geometry size multiplied by a factor determined from objects Scale property.

They are a user-interface convenience to show resulting object dimensions rather than just the actual Scale property of the object, as such they can't be animated directly.

To have size change over time animate the scale of the object instead by keyframing Scale X, Y or Z.

If you still need to obtain accurate dimensions or particular sizes, adjust the intended dimension so you get the length you want on that specific axis, let Blender calculate the corresponding Scale factor automatically, then keyframe the resulting factor from the corresponding axis Scale property instead.