If it's this important, does the onboard computer of the car not check this and warn if needed? If not, why not and why did humanity end up in a state where clunky apps are needed for this when the car already has all the electronics it needs for this?

I mean, probably nobody looks at whether a stick to measure the oil level is integrated in the oil cap either, but it's still there fortunately (or do you need an app for that too these days?)

Unfortunately, a lot of vehicles these days, mostly high-end, do not come with oil dipsticks anymore[0]. I know that Audi starting switching to sensors-only around 2012, and I believe BMW started even earlier than that.

[0]: https://www.motorbiscuit.com/oil-dipsticks-disappearing-at-a...

People are accustomed to the coping mechanisms: aggressive battery replacement (on a schedule / at the first sign of struggle) or letting the battery deteriorate until the car fails to start and then asking for a jump / calling a tow truck. People see these inconveniences as inevitable, not as the result of a missing feature that really ought to be standard. Manufacturers don't feel pressured to do better so they don't do better.