In several episodes, you see that the TARDIS has a new console. Why doesn’t it just fix the old console instead of building a new one?
The look, feel and even control systems of the TARDIS seem to change according to the whim of the current incarnation of the Doctor.
In the minisode "Time Crash", the fifth Doctor compares the control room's appearance to that of a computer's desktop theme;
FIVE: What have you done to my TARDIS? You've changed the desktop theme, haven't you? What's this one? Coral? It's worse than the leopardskin.
Additionally, in the episode "Eleventh Hour", we see that the TARDIS itself is capable of selecting a new Control Room when the previous one is destroyed during by the Doctor's regeneration cycle.
As to the specific question of why they don't simply fix the old one when it gets broken, the answer is that the TARDIS contains a considerable number of "themes" from both the doctor's past (and future) that can be selected at will. Changing between them seems to require a trivial amount of energy compared to fixing a broken one.
DOCTOR: How's he going to be able to take down the shields anyway? The House is in the control room.
IDRIS: I directed him to one of the old control rooms.
DOCTOR: There aren't any old control rooms. They were all deleted or remodelled.
IDRIS: I archive them, for neatness. I've got about thirty now.
DOCTOR: But I've only changed the desktop, what, a dozen times?
IDRIS: So far, yes.
DOCTOR: You can't archive something that hasn't happened yet.
IDRIS: You can't.
If you'd like a breakdown of the various control rooms seen in the TV show, there's an extensive list (with pictures) here.
It's stablished in I don't remember which chapter, that the control room of the TARDIS changes to match the personality and taste of each incarnation of the Doctor. The TARDIS does this automatically and has stored every main control room that the Doctor used or will use. That would be the "canon" answer, but it´s really an excuse to keep updating the scenario to match the times in which the show is aired and the particular taste of the director of the season.