The mounts are not *completely* removed because they double in the original frame,and the modifdied one, as skid plate mounts.
So to cut them off, and then reweld new ones doesnt make a lot of since in the big picture. Same with entirely making a new cradle,
when it isn't needed.
i do so on certain years of the Honda 500AF models, others years no.
It depends on the particular frame and particular need.
it is a production bike - not a one off.
costs /strenght and many factors are involved in each detail or decision.And a big effort is made to keep these bikes in a somewhat reasonable price point.
The cost of the bike is mostly generated by the cost of a brand new KX500 engine.
The engine alone is 6K to 7k, depending on your partcular OEM pricing on the KX500 parts.