This very much depends on the case. If you have an intake at the front say and you will have an exhaust fan at the back then in normal circumstances extra cooling is probably not needed. But if you have a high end GPU and play graphic intensive games then extra cooling by way of another intake and perhaps an extra exhaust fan in the roof is probably going to be necessary.
Likewise if you are into over-clocking your components,then this will produce a lot more heat and your cooling must reflect that.
As to why PC manufacturers not include extra fans, most of the time it is not needed, as for PC case manufacturers then some come with a lot of fans,I had one of these,Silverstone FT02 which as you can see came with 4 fans, I have a Lian Li that came with 3, and I also have a Fractal Design case that came with 3, it will depend on the quality of the case as to whether you get the extra fans.