Why aren't petards more like demos?

I'm watching the MSC tournament (https://www.twitch.tv/videos/455426880) and it looks like a lot of fun. But it makes me wonder why petards aren't more like demo rafts. I think it could work well if they were more like 60% damage of a demo raft (so some aoe damage but not enough to fully kill an upgraded archer) and had bonus damange to buildings.

Currently, I don't think anyone uses them for area-of-effect damage against units, and only very rarely does someone use them against buildings (and even if they do, it is only against walls, castles, or tcs).