Interesting. I never thought about looking at it this way, the parts. But then I also noticed that it is not uncommon, rather actually common, that Lego uses at least one never-before-released part in new sets, usually a part that exists but with a color that never existed before but also sometimes parts that never existed. I don't know how difficult and costly it may be to reuse a part that was "retired" but it should not be worse than releasing a brand new never before released part.
The thing is that I believe there is a more than big enough amount of Lego fans that would buy those modulars because they became too famous and that Lego would never lose money, on the opposite, they would make more money in each of those sets if they were re-released than probably they make with new smaller sets that they keep flooding us with. These modular buildings are premium sets, they are expensive, they never get discounted, which means they give Lego high profits.
Also, they don't need to re-release them exactly as they were, they could well adapt them to current molds and they would still look just as nice. They could release improved versions of them, buildings that look similar but have better interior and small adaptations for missing parts that they don't want to bring back from retirement.