On Twitter it has been said that "browser-vendors support web-standards when and only when those standards align with their own business interests". That's not always true, and even if it was, "business interests" are broad enough to make surprising results possible in a competitive browser market.
Mozilla, as a nonprofit, isn't entirely driven by "business interests". Mozilla often acts for "the good of the Web" even when that costs them money. (Example: pressing on with their own browser engine instead of switching to Chromium.)
Other vendors perceive (rightly or wrongly) that being seen to "do the right thing" has some business value. There is a PR and marketing effect, but also a recruiting effect; being seen as an evil empire makes it harder to recruit talented staff when other good options are available. To some extent Mozilla's existence has encouraged other vendors to compete on virtue.
Competitive markets can force vendors to implement standards they otherwise might not want to. For example, Apple needs an iOS browser that can render the modern Web or they'll leak market share to Android, so they're forced to implement Web platform improvements that you might think are not in the interests of their App Store business.
Decisions about Web standards and implementations are often made by individuals keen to "do the right thing" even if it might clash with corporate priorities. Everyone's good at rationalizing their decisions to themselves and others.