It's the same reason I don't race online in a tuned Clio or a Honda type R Civic.
It's because they have an advantage.
And it's not about winning with the easiest car, it's about beating it with an inferior car.
The simple fact is you're not given an inferior weapon, you're just given different ones, the early weapons, SCAR, M4, Famas, are some of the best, the best sniper rifles are available from the start, arguably the best shotgun, the SPAS 12 (outside of akimbo) comes early, the most valued perk for most core modes, stopping power, along with similar high value perk steady aim. Game for game the level 70 has little advantage over say a level 10, as proven by many of those who have prestiged, they don't miraculously score worse and they don't magically get better. You have less options, but those available to you are some of the best and certainly work and are popular all through the levels.
If it didn't, then why is it there?
It may not be any great improvement to prestige, but it seems to me as natural progression.
It's there for players who see it as a progression which it is not, like I previously said, it's like re-doing campaign, you're nowhere where you previously haven't been. It's repetition bar the rewards of badges and custom slots which as previously pointed out don't matter to everyone, some people like to (and here comes the big shock) just play the damn game and don't care about ranks, stats, prestige, nukes and so on. Playing the way thats most fun to you, like for me a silenced assault rifle toting stealth guy, by no measure means you're scared of prestiging, quite frankly it's plain stupid to make that assumption and insulting to those who just play to have fun and don't want that shiney new badge and realise prestige is repetition not enhancement. In reality, something like a max level prestige 10 doesn't say, good player, it says experienced, they play alot but it doesn't mean they're good at it.
Surely a natural progression would be improvement, gaining things, not losing them, 1 2 3 4 5... 1? Going back to where you were before is not progression, if it was a new situation perhaps it would be, but it will be where you were before but made easier. It's an option with it's rewards but it doesn't enhance the experience as you actually play the game, for me it makes it worse because of the way I wish to play, for some it makes no difference and they play as they would anyway but it never enhances each game, round to round as you play. This is the key behind many peoples reasoning, the game doesn't get more difficult by prestiging, it just doesn't as I've seen many players, some here at GTP, hit prestige then play their first game and do just as well as the one before. Said player is not miraculously having more fun and thats what games are for, enjoyment. Some people want to take every option and do everything others just don't, ever.
Some people played GT4 to a point then stopped trying to progress, started doing other things, photomodes, tuning cars etc. Are they scared of moving on or are they just playing how they like to enjoy themselves? And with GT4, they'd geniunely get new cars and things if they did continue to progress, not just regain things they already had.