As far as I understand it, it’s generally a genetic defect. Many cultivated plant varieties are specifically bred for partially white leaves, because it makes interesting-looking garden specimens. It definitely makes them a bit weaker, but in cared-for garden setting it’s not really an issue.
You’ll find completely albino plants surprisingly often, usually if you look at seedlings of large-seeded plant species. The small seeded species use up all the energy in the seed before they get big enough to get noticed, and then die. But things like California Buckeye, for example, can get 6-8 inches tall before their seed is depleted and they die, so they’re more likely to be noticed.
Here’s a couple I’ve found over the years: https://www.inaturalist.org/observations/10064790
and https://www.inaturalist.org/observations/7810326