Cause its not that uber.
Power resides where people believe it resides. Conversely, the best builds are what people believe they are.
So I am doing 500+dmg games (pugs, admittedly)... with a hunchback. With zero gauss zero ppc zero ac20s.
I am using a hunchback 4P loading 4 LLasers. Since its not "THE" meta, I go largely ignored in-game. (it helps that I try to stay out of the way, and make myself a difficult target).
And since the lasers cause no cockpitshake to the target, they dont generaly return fire, as I'm just a minor annoyance.
Until I start ripping off limbs, that is. Or CTs. Or Legs...
Also, avoiding snipers at long range can be quite a sport. Just move erratically, perpendicularly to them. Go, stop, go stop and see rounds flying everywhere. Dont charge face first into a sniper nest. Or better, calmly go around, and shoot them from the side. Or rear.
My Atlas RS is also loading a pretty weird config: 4 LLasers (sometimes 2ERLL and 2 LLs), and a couple of... LRM15s.
And I discovered (after trying a build with 4 LLs and 1 gauss), that the LRMs are actually pretty awesome. You just have to use them right.
LRMs are not "first blood" devices. They are meant to be used when the brawl really starts, and people begin to engage earneastly. Wait until some shooting matches have commenced, you can start making it rain with relative inpunity.
Another role I have been happily ding is taking a ECM commando, load it with an ERPPC (ergh, but for this its usefull), and use it to light up targets for the lrmboats.
I take the erppc so I can shoot some people with no trace (no long blue beam signalling exactly where I am), and since its a ERMAGERDPPC, the entire team turns around to deal with the OMGPPCBOAT. Then I dissapear, sowing havoc with a lowly 25ton mech.
Then again, this DOES require DESPERATELY some tonnage balancing: Either
a) Matchmaking requiring both team's total tonnage be only 10-15 tons of total difference (maybe even doing 8vs 6, if its 6 assaults vs 8 mediums/heavies)

c) Give a money multiplyer by weight class. Giving the same c-bills for damage done to a stalker (which only has to sneeze at someone's general direction to rake in 100 damage) as to a commando (which to rack up 100dmg has to be shooting for 8 hours straight). Is skewing mech choices towards heavier mechs. Especially since lights and mediums can be a bigger asset to a match than an assault mech.
Give big bonuses for spotting (not just for LRMs - give the bonus when you are lighting up the target on radar, reporting the location and said target gets hit by anything), scouting (first to spot a mech).
Change the scouting bonus to be a LARGE % of the damage done (so a light lighting up an atlas that consequently gets killed gets a ton of cbills) - this would encorage lights to actually, like... SCOUT and bring BAPs to the field.