You forgot they are trash with damage output

Their limited availability gives them a disproportionate value.

They was in the first year of mech arena meta, but in evry of the waves they aren't made bester yet.

They use to be better, but over time more powerful weapons/mech came out. I believe they also got a nerf at one point too like they end up doing with lots of weapons after the cash cow effect of them wears off. It goes release a new weapon with OP damage, cash in on the people buying it, let everyone whine who don't buy one, Claim it's been tested and everything is fine, after a while(when sales havealmost stopped) say that after more testing and player feedback that they are working on adjusting said weapon, claim a fix is in thw works, nerf the weapon, release a new powerful weapon and rinse and repeat. This path has been done many many times with the other weapons. 

I think Carbines were the first legendary weapon in the game.  They simply haven't been updated to keep them competitive as new weapons come out.