Why can they not be installed on top of existing deck? I was advised to do so by a builder?

Tropics composite decking, or any similar wood plastic composite decking should not be installed in this manner. If applying new decking to an existing deck, the old boards should be pulled up, the joists and structure below must be reviewed for proper spacing, any need for replacement due to rot, and the nail holes in the existing joists should be sealed. Wood plastic composite decking does contain wood in the core. It has natural thermal expansion and contraction, and despite being a capped composite, should not be installed with less than 6” between the decking and the ground. This would include avoiding installation of the decking directly over existing decking - natural wood, composite or otherwise. Tropics decking should be installed on joists that are spaced 12” to 16” on Center.