I don't think it's Lego being cheap, I think it's Lego knowing what price point they can get the consumer to pay out for a product. For Gru's house, you'd be looking at $200+ to turn that into a modular style dollhouse and is any Despicable Me consumer seriously going to pay that much?
I think Malfoy Manor has a similar issue: how many people were going to pay $200-$300 for a modular dollhouse version of it. It doesn't even appear very much in the movies and only a handful of scenes take place there.
Then you have a second issue: the scenes that do take place there involve murder, kidnapping and torture. So already the set is less kid friendly, alienating a large section Lego's usual consumer base. So you have a set that's basically more palleteable for adults, but also is not a popular location from the films. So was Lego really going to put out a high priced set for this? It just feels like a modular version of Malfoy Manor would have been a big gamble.
I actually really like the set the way they did it. The facade is flat, but so is the actual facade of the building it's based on, so that works for me. If it has the long table and a few other things like a dungeon, balcony, etc then I'll be pleased with it.