Is this common with switching regulators?
Why not use an "integrated version" of switching regulators which do not require such a large number of different types of external components.
Not possible for this current and power range with dual outputs.
In order to improve cross load regulator noise, separate converters are used.
Consider that every part has a design purpose that for better or worse was the most cost effective choice if the design is in high volume and has high quality specs. The design you show is one of the best buck regulators, a Half-Bridge feedback ratio variable voltage buck regulator with fixed parts.
In order to get the best bang for the buck, it is not feasible to combine all the parts into silicon.
The concepts start simple then get more complcated with low error and high power specs.
Small lithography analog and digital ASICs cannot do low RdsOn MOSFETs on the same wafer. Nor can they do large reactive parts or large current shunts.
Consider : high energy storage in LC reactive parts and high current in semiconductors, input transient OVP, output current limiting , output LC ripple filters.
Consider the specs for your DC-DC multiple output converter.
12Vin, Output 1.2V,3.3V, total 27W output <1% Voltage regulation <1% 50% load regulation error, < 2% ripple, crossload regulation error 0.5% , low EMI, etc etc.
Can you think of a better way? Look on your MOBO to see how little space it takes to supply all the internal voltages for RAM & CPU , all controlled by BIOS. Locate the coils near the CPU.
Then for AC to DC converters we have lightning transients and HIOT isolation requirements. Not every design is scaleable due to I^2R exponential demands.
But here is a simple AC-DC converter.