That is because there are so many different metals in a modern engine. If the Ph is too far out of whack, galvanic corrosion can eat things up. That is what gives the odd stain patterns on shafts and surfaces that are together a long time. The Ph of the oil is really affected by the water it picks up because the water will react with gasses to form acids. That is why lots of short trips are bad, lots of blowby and gasses and lots of moisture that does not have time to be driven out.
Oil starts out a bit on the basic side and gets more acid over time.
I think they assume that most cars get a fair number of short trips. Ph will corrode aluminum and an acid will corrode zinc and iron. I use this as a justification for not changing oil too frequently. This a place where a lot of compromises are made and specs are pretty vague. In general, filters are rated by the pore size in microns.