They essentially can be. All you need to do is write in the 'Rage' section "The barbarian can cast the Rage spell a number of times per day equal to the Rages column". And just translate the Rage's mechanics into the spell block format. Voila! Rage is now a spell, just cast in a different way than the spell slot format the other casters all use.
This is why I've never gone along with the whole "too much magic in D&D" complaint... whether something is a spell or not, or magic or not, is entirely just down to how the mechanic gets written in the book. You write the mechanic without using the word 'spell' or 'magic'... then the ability isn't. You write it with, then it is. And if that's all it takes... just reading how an ability or feature is described... then I simply just change how it's written for myself. And thus if I need a Warlord type of character, I use the Bard class chassis and just change what was written to describe what these features are-- swapping out 'spell' and 'magic' for a pair of non-magical words instead.