Because, to use the same logic as I provided you for the transmission, in the case of the engine oil x approaches zero, and y is significant. Engine oil is designed to be drained, a drain is provided, draining doesn't cause other problems, etc. Not changing engine oil impacts engine life. The fluid doesn't wear out (oil is slippery for its entire lifetime, as I am sure you know) but it gets contaminated by byproducts of combustion, and the additive package gets depleted. Comparing engine oil to transmission fluid is nonsensical. It would be much better to compare transmission fluid to shock absorber fluid, they are both hydraulic oils in sealed systems. You could change your shock oil I suppose; many years ago we used to rebuild shocks, particularly lever type shocks. Now they have lifetime fluid in them. Not your lifetime, the shock absorber's lifetime. Maybe that is a plot to get us to buy more shock absorbers.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue