WHY ? ? ?
Well, a few of reasons.
Each engine type has a different spec required with different additives to achieve those specs. Example is oil for my Harley Road King. Twin Air cooled engine requires waaaaay different oil that an automobile. (and actually when I looked this AM, it takes 20W50 syn.) Harley air cooled engine requires JASO MA/MA2 rating. Also, many guys also run this same oil in the Primary and Transmission (since bike has three separate holes). I do not and run separate Primary oil in Primary and Gear Oil in Trans. What I am trying to say is, every different type of engine has different needs. The Harley is extreme.
The Volvo Penta stern drive engine in my boat is a cast iron block (GM 4.3L Vortec block but marine so cast iron) and has some different needs than a regular aluminum block car engine. Plus, it is marine duty, thus has different moisture handling characteristics through additives, and no catalytic converter, hence quite a different rating.
My 2018 F150 takes 5W20 and I use synthetic Mobil1 as it is a very good oil. Why 5W20? Fords says so. These newer engines are built to higher spec standards, better tolerances and require a thinner oil. I use syn because I can.
GTractor with Kohler Command, I use Amsoil syn, again...because I believe it to be a better product and I can.
So, I have a lot of $$ in my engines, vehicles and toys. That boat alone is above $90K so why the hell would I run a cheaper oil in that from what is spec'd? Harley with my mods on it is over $30K. Again, why run cheap one-oil-for-all from a 5 gallon bucket I buy at TSC in it?