Tracking, mixing and mastering are all different, logistically, sonically, all different.
They are all different trades, requiring different tools and experience.
Analogy: Framing, drywall, painting. Yes all 3 things come together to make a wall... but you wouldn't want your painter to do the framing, or vise-versa.
However...
If you are experienced with all three then there really is no problem doing them all your self. In fact, if you ask me, if you're REAL good with all three, then you will get the best results possible since you know all the inner-workings and have a goal/vision of what the final product should be. BUT it is always nice to have another person's ears, brain, ideas, experience, gear, room, to influence the music in a way you may not have known possible.
If you are not well versed in any aspect of producing a song, it's either best to: practice, practice, practice, and gain experience (maybe 5-10+ years). Or hire a professional with experience.