I'm not sure we can always interpret 'optional' as 'nice to have but not necessary'. Optional fields can often cover edge or alternate cases like an apartment number in addition to a street address, a dietary or other accessibility accommodation, etc.
You could work around it by adding a bunch of conditional fields which then move more work onto the person filling in the form (tell me you need the extra fields then I'll show them and they will then be required) and add code complexity (more UI flows to test).
Many optional fields are indeed nice to have but assuming they all are misses that the world has a lot of fuzzy edges, and optional spaces handle those with a simple convention that many people expect and understand.
Roughly the same, I think the variance is about 10% with Steam being an outlier working in tiers tied to revenue. At least one allows developers to name the cut (including 0%) but 30% is the most common number.