What was the point of not declaring war on ukraine and calling it a special operation?

Legality. In russia law. When there is war. Rules change. Stuff like draft goes in effect. They wanted to avoid stuff like that espesily when plan was for quick thing.

Soldiers sent to war on foreign soil are being paid extra, but if they announce all of Ukraine is Russian then they are stationed inside their own country while also technically not at war, so the wages can be kept low.