Because once it starts getting acidic and wet it will age out. Like washing dishes, you can try to use a lot of dish soap and water but it is better to dump out the dirty stuff and start from clean water and dish soap every meal, instead of letting the dirty dish soap and water sit till the next day and double-dip it.I've noticed that as car motors shrink, so does the oil capacity.
So why can't you just double oil capacity to double or triple oil change intervals? Engines seem to have such small sumps that the oil is cycling continuously shearing it down faster.
Now stick a 6 quart sump on the 1.6 liter engine, you could get 14k+ OCI's since you got double the oil capacity, and a high mileage oil filter.
It will also take a heck lot longer to warm up double or triple the amount of oil.