Because unlike features in the player’s handbook, you don’t get the Steady Aim feature automatically. You have to consult with your DM, who will decide whether or not you gain it when you reach 3rd level.So why does it say this?
This section offers an additional
feature that you can gain as a rogue. Unlike
the features in the Player’s Handbook, you
don’t gain the feature here automatically.
Consulting with your DM, you decide whether
to gain the feature in this section if you meet
the level requirement noted in the feature’s
description.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.