As for the reason why: my best guess would be it has something to do with the technical experience of the average user. An escrow agent has a lot of responsibility and has to deal with inexperienced users asking "newbie" questions all the time.
I can see this being an issue if the users had to construct the multisig address themselves. Why don't people use Bitrated though? Do people not trust that it's constructing the 2 of 3 correctly? Or is it still too complicated? Or do people just trust the arbitrators on bitcointalk so much that they don't see the value in having even more security?
So however it works for escrow, a guy, a company, you still got to trust them based on the community feedback for them.
Sure, if you use localbitcoins then you have to use them as the arbitrator. But let's say you use something like Bitrated -- then you can choose anyone as the arbitrator, so you can just use the same people who already get used as arbitrators on bitcointalk. It just makes it impossible for them to steal the money without the cooperation of the seller too.
I guess it just blows my mind that after all this time we still aren't seeing people use what I think is one of the coolest features of Bitcoin (2 of 3 multisig) when it looks to me like people should have a clear incentive to use it.