The reasons lie with the spells. Those scale in power qudratic or even more with spell level (meaning 81 lev 1 spells are not as useful as 1 lev 9). With level 9 spells you can destroy entire armies or similar powerful effects.
If the wizard of the party in the long run gets that powerful for pure fairness, the fighter has to be powerful in similar way.
But take away items, what is left of a lev 20 fighter.
Dam 2d6+24, Bab +16/+11/+6/+1, AC 25, flat foot 24, 195 hp(using combat expertise and power attack).
If such a guy challenges the army, they laugh about him, surrond him and the lev 1 warriors attack with effective +6, having a 15% hit chance for 2d6+3 dam. Meaning after just about 130 lousy losers (CR 14), he is dead.
On the other hand the wizard can make himself invisible all day and send the summoned monsters till they are dead or just blast them with some fireballs.
And the lev 20 fighter without items has a will save of +8 at best, so any lev 3 wizard with blindness or other nice spells has a decent chance to render him useless.
So either level bonuses or only low level playing(there you dont notice the power difference so much) or different system, where the wizards dont get city leveling spells.
Why wouldn't the fighter use his feats? Great Cleave and Lunge alone would make him awesome in this situation. Combine that with many other feats and he would do just fine. Sure, the army might eventually take him down but who wants to fight a guy who is going to take out 8-30 (more if he is already using a reach weapon and quick draw would let him switch as needed) of them every 6 seconds?
I understand the point you are making. Magic > mundane especially at high levels. Gear is very useful at keeping the characters alive. This becomes more and more true as the characters level. Even the wizard is going to need some magic items if he wants to remain effective, if only by increasing the DCs of his spells.
The best way to avoid the Christmas Tree Effect is to not require it for characters to do well. Don't have the characters battling the most potent enemies all the time. Use more of the lower challenge rating opponents. It makes for a more interesting battle anyway. Also, don't use so much combat. Increase the roleplaying in the game and provide XP for overcoming obstacles by using their skills instead of their equipment.
Give their more powerful gear charges too. So a magic sword would have 20 hits per +. Once it uses 20 hits, then it is reduced by one +. A +5 sword would be a +4 sword after 20 hits, +3 after 20 more, etc. Save the more potent creatures (ie dragons) for when they might need to pull out their big guns.