Well, usually they'd represent a great deal of real-time and game-time during which your character was interacting with other PCs and various NPCs, forming friendships and enmities, achieving goals, finding new aspirations, expressing and developing various traits and personality quirks, and generally developing into a true individual.
To me (and I accept that YMMV), that is what makes a character truly come alive, truly become valued - their interactions with others. In many cases, the character will develop in a completely different way than I envisioned, and usually be all the better for it.
Compared to that, the few minutes spent putting numbers together on paper, and even the few hours of periodic idle contemplation that go into creating a personality and backstory, are a miniscule investment, and one I can easily replicate in my own time. Creating a brand-new character is a piece of cake - but creating anew a place for that character within the party and the gameworld is a far larger investment.
Putting the numbers down isn't that hard at all, but I must be investing more into character creation than others. I go ahead and decide goals for the character, why it is adventuring, what it hopes to accomplish, etc when I create the character. Not just looking at it as the numbers on the page, but that is the last thing I do after a good while of creation for the character.
Why am I playing the elf? It isn't for some bow bonus, or something else in the mechanics. What does this elf think of others that he might meet, etc.
I design my character then worry about the sheet. Therefore the character has had much work placed into it from the start that is more than just leveling could do. the character will grow and change over tie, but you have to have a place to start from rather than Character X and after a few adventures he gains a personality and such. They are already there from the start. Adventuring adds more to the character story, and the levels just alter the character sheet.
You what you do during game time to gain those levels is the reward for that work. A beginning character has all the work and no gain so is of greater loss.
@Jhaelen: It isn't always the character that bores you, but the routine of the story sometimes. That is where little breaks for one-shots with a new character for a single session help to break the monotony, and maybe give the DM some recharge time so the adventures don't get stale.