It's an anti-bot measure from back in the ARR days. Bots used to just sic their tank pets on nearby mobs for exp farm/certain valuable drops and keep them alive with Sustain. Didn't matter that that was slow as balls when you're a bot, it could be 100% automated with no real danger to the bot character. Because of that, SE changed it to where the player needs to actually aggro the mob or object in question as well (didn't do much against ACN bots who would just cast a single Ruin II on everything in sight and have their tank pet AOE it all up, but an attempt was made).

Having your pet be the sole source of aggro will not only skimp you on quest credit, but exp and mob drops, too. At least tag it with a dot before letting your pet go to town.