Originally at the birth of indoor plumbing, they installed a cold tap,and as most people were right handed, it tended to be on the right. later on when they were able to produce hot water by thermo-syphon from a back boiler, and a storage tank in the attic, they installed the hot tap in the only other available space , the left. and of course they were delighted, as they did'nt have to disturb the original cold tap.
This scenario extended to after the Second World War when designers decided to incorporate the two, usually in baths/tubs.
They then decided to make monoblock type suitable for sinks, but as they could only use smaller pipes ( 10mm 3/8" ) to connect to the H & C they discovered that the flow rate was reduced, and that is what we have today, a reduced flow in the kitchen sink,
OH FOR THE GOOD OLD DAYS.
In the UK the pressure generally does'nt exceed 2 bars (30 psi ) and even today there are many mains which are lead pipe, so they cannot increase the pressure.