The thing is that phones can be lost, robbed, broken more frequently than a Desktop. It is more a safety thing than a security thing I think.
This. Even if I was sure that online wallet is secure and never will be hacked there is always possibility of physical robbery, you can drop your phone, soft update can go wrong etc.
There is no such problems with stationary PC. Even I thief broke to my home (highly unlikely) he would take my laptop or smartphone first, because my PC is rather heavy and big.