Because offchain would not be protected against double-spend.
So now we are using their central servers to process transactions off chain, why is their system vulnerable to double spends?
This off blockchain server supports sending payments to every single user, merchant, service provider on the planet?
Wow that is both amazing and never going to happen. If EvilCorp has 20% of the network hashrate then sending a payment to anyone not using their centralized servers will take 50 FRAGGIN minutes for first conifrmation due to witholding the the tx from the other 80% of miners.
Yeah I see that service being "super popular". Even if people don't care about the network security they certainly don't want massively delayed txs just so evilcorp can rule the blockchain.