That's because they are one of the most global bank, therefore a prime choice when it comes to moving money from countries to countries. Most bank have a much smaller global footprint and can't be used as such.
Banks don't benefit from their customer laundering money just like landlords don't benefit from drug trafficking in their building: it's a hindrance and it costs a lot to do anything about it.
Source: I work on AML in a global bank.
