In a Class Action Lawsuit, the law firms cash out big-time if they win. The law firm gets a percentage (often 1/3) of the total settlement, so that's a strong incentive to bring these suits and make them as broad as possible.
BUT, I don't know if arbitration proceedings have the same financial advantages for the arbitrator. That could be a Very Important distinction. I strongly suspect the reason this is in arbitration is the contract terms between the advertisers and Google force arbitration to settle contract disputes.
Arbitration is to try to prevent class action lawsuits. Basically class actions are costly because of numbers - if you steal $1 from many people, 99.9999% of them will not do a thing about it because a lawsuit will cost more than $1. Class actions solve this - because if you know a million people, suddenly a million dollar payday is a lot more worth it to pursue as a lawsuit than a million $1 lawsuits. Of course, it would probably cost $250K to do, so you'll get 75 cents on the dollar. Though usually you want punitive damages so the company doesn't try it again, so you might get $2 after getting ripped off $1.
And lawyers are often not part of the compensation - if they reach a $200M settlement, that $200M is almost all going to people of the class - the lawyers getting another $50M (remember they have to front all the costs upfront) is outside the settlement. Then the lawyers need to make arrangements to distribute the payout which is a lot harder than it is - whether it's printing cheques, or doing digital deposits.
That's why companies went with arbitration instead - because now if they screw you out of $5, you're not likely to bring in a case for that and if you're not doing that, they can keep that money.
The way around this was accidentally discovered a few years ago - they realized that companies were relying on the fact that consumers weren't going to bring cases against them with arbitration, but suddenly if you can get a sizeable group, the arbitration fees can be exorbitant. And since the company has to pay for it, this can be rather expensive.
So some lawyers started specializing in mass arbitration where you can get thousands or more clients together to individually argue their case. If it costs the company $100 per case, suddenly they can be out thousands in arbitration fees in the span of a month. And if you do it right, the arbitration company might get slammed because they probably are to handle maybe 10 cases a day per arbitrator, and suddenly having 2000 cases presented presents a huge issue.