Advantage of series vs parallel battery in handheld product
The decision to use a particular voltage input is not necessarily made on the major supply voltage used in an appliance.
Consider your question around the ASUS laptop: 

Its input is 19 V; a fairly standard laptop supply voltage, but not likely used for anything internally compute or peripheral related. 

@19 V and 45 W you can expect about 2.3 A maximum line current.
@12 V, that would rise to about  3.75 A.
@5 V ... 9 A.
@3.7 V (1 cell voltage) ... 12 A.
Voltage loss would become more critical in the wires and connectors at lower voltages making overall design much more challenging. It's also much more challenging to design an internal battery charger since you now have to separate input/battery at the same voltage with FETs to allow a SM charger to operate. 

Internally they may not even charge the batteries at full current when the laptop is on at the same time to control both the power dissipation within the shell and current magnitude in the input supply line. You can see from the specs that the compute/display side of the laptop is only 5 -7 W depending on display usage (resolution/brightness).
The battery in the ASUS is likely a 4 cell 14.8 V battery with active capacity management but no cell balancing. I don't see it specified, but would assume about 1 - 2 hours maximum charge time for the battery at say 2C. 

Most of Intel's reference designs are based around a 12 V supply (Core level compute elements). To abandon the reference design (which is fully debugged) and design a new way to do it would be a risky undertaking. You rarely see OEMs such as ASUS, Apple or Microsoft stray far from the reference platform in anything other than peripheral devices.

So back to your question ...could the laptop be powered by a single cell paralleled battery pack ....sure, but it would be significantly harder to design the appliance.