I'm obviously not a ship designer but cargo ships are built for massive internal space (i know right). They are usually built like a cheap plastic bucket (with emphasis on the rolled up lid), that means they are still quiet bendy and probably make some noise if they are older. But I mean if your megaproject living space incorporates hobbys needing a bunch of gym sized rooms, yeah you could do it. But it would probably be structurally challenging to put like giant hatches or windows into the cargo bays and it would take major redesigns to cut down the hull.
With a general cargo you also get very shit top speed, they are basically designed for a certain speed but if you're superrich that's not really gonna bother you, I assume.
I think nobody does it, because Container ships are 100% marine grade steel plate and every aspect about them feels heavy industry and not at all yacht'y relaxing and most individual owners of commercial vessels use them rather for their intended purpose because that's what they are interested in. Like Clive Cusslers NUMA ship and nowadays the RV Petrel, the usual arctic exploration boats and so on.
What about roro ferries? They feel halfway yachty because they are built for people and have a deck that's pretty much tailored for a lot of hobbies already..