I understand that lower viscosity oils are way easier to pump and move around engine parts, also they form a thinner "film" on friction surfaces which could improve parts measurements precision etc. Is it only due to increased piston rings leakage that we don't use extremely thin oils? Are they more prone to failure in operating and high temperatures? Are these oils easier contaminated? Maybe they are very expensive to produce???
Thin oils tend to have a very poor shear stability, especially when they are hot. The function of the oil is to protect and lubricate the engine and ancillaries (i.e. turbo chargers). In certain applications it also makes up some (where an oil cooler is employed) or all (for aircooled engines such as the Beetle) of the circulated liquid cooling system.
I'm not sure that an oil which is consistently thin and remains so once upto operating temperature exists. Most engine oils are chosen so that they will provide some protection at cold start but optimum protection when they reach their target temperature.
Tribology is a huge topic...
But, the thinner the oil, then the more oil in terms of volume and/or the higher pressure is needed for a given tolerance between bearing and surface.
So many things are taken into account when specifying the oil for a particular use. Operating temperature, load on the bearings, relative speed of the bearing etc etc
We are, or are starting to, for exactly the reasons you quote.
New vehicles (VAG group for example) are starting to use 0W20 oil, with 0W16-capable engines coming soon and tests being conducted with oil as thin as 0W8.
Source: MTZ worldwide 12/2018, "Potentials and Risks of Reducing Friction with Future Ultra-low-viscosity Engine Oils"