compressed storage would bring huge performance and size-on-disk wins. I suspect the issue is the difficulty and resources needed to add compression on top of the existing design rather than a decision that the tradeoffs are not worth it.
The problem with heap and compression is that it mixes a lot of different data (i.e. data from different columns), which is not great for compression ratio. To address that, it's necessary to use some variant of a columnar format, and allowing such stuff is one of the goals of the table AM API, zedstore AM etc.