That's because these tubers cannot be used for reproduction.There is some question what they are used for! But these tubers do not have the ability to 'make new plants'.

Here is a link that might be useful: Definitive on Mandevilla tubers!!

Thank you. That was very helpful.

I suspect that they are storage organs that act as reserves for the plant. Similar tubers on plants such as the South African Asparagus densiflorus store both starches and water, imparting drought resistance to ride out long dry seasons in habitat. This enables the plant to be successful in coping with wildly eratic seasonal rainfall from year to year, and is a characteristic of many plants from seasonally dry subtropical climates. As Mandevilla also comes from parts of Brazil with seasonal drought, it may serve a similar purpose.

This has been very helpful to me as well. A friend of mine has been asking about a tuber that is above the ground. Now we know. Thank you.