They're cheap, addictive calories. HFCS and sucrose also don't trigger our satiety responses well, so that also allows us to eat more than our body needs[0]. It's a battle over space in our stomachs, and sugars are a massive weapon in that competition. Food science is practiced by food companies not to make us healthier, but to ingest more.

[0] https://www.ncbi.nlm.nih.gov/m/pubmed/18065574/