If they were essential don't you think that the manufacturers would put them on in the factory?
I hear this often and each time I’m reminded how many car enthusiasts don’t understand automobile engineering and manufacturing or, for that matter, manufacturing in general.
Without getting into a debate over the merits of a catch can (I use them on the passenger side of all my modular motors), your statement is completely at odds with the realities manufacturers face. They are required to meet numerous benchmarks, many at odds with each other, and you cannot simply say that the omission (or inclusion for that matter) of a part is an optimal solution.
You intuitively know this. The most obvious examples are the trade offs between performance, durability and emissions. If you are making a car and are required to warranty the emissions system 4x longer than the powertrain, which system are you going to favor over the other in terms of performance and durability? Cost considerations are another huge trade off. How many new trucks don’t have a differential drain plug? Is the absence of the plug indicative of a “best” design or perhaps a $1 cost saving measure. You be the judge.
The examples showing why your statement is incorrect are numerous.