I think the "full node" philosophy is to have all data uncompressed to be fully available. I don't know how it works specifically but maybe they need to check every inputs/outputs to broadcast transaction ?

Some altcoin are doing pruning or "light nodes" but I don't think it's very good for decentralisation.

And compressed data = less cold storage space, but more processor work when you want to access old data. So less data for more computing, and data are cheap...