Why LM2596 buck converter output current is low when used as a mobile charger?
Phones do USB power negotiation to see what current the USB is capable of providing; your buck converter doesn't negotiate so the phone assumes a maximum, below what you expect.
My older Samsung mobile phone is happy when I short the data lines D+ and D- in my home-built charger's USB output, but this probably won't work for all phones.
If you want this to work for all brands and models, you will have to use a USB Charging Port Controller like the TPS2514, but in view of the USB spec changes, and brands being idiosyncratic, even that may fail for some phones.
Here's some more reading, in addition to the Mintyboost write-up Chris Stratton suggested. Note that the USB 3.x specs have some new ideas about all this.