Because what they want doesn't correlate with what's in demand at that point in time?
I mean, someone might want to study English Literature at degree level, then find a job based on those skills. However, if those skills weren't in demand at that point (because everyone wanted people with STEM degrees or programmers or what not), then their outcome would likely be worse than someone who studied computer science.
Did both of these people have equal opportunities? Sure, they could have both gone into one field or another (assuming a theoretical situation where their intelligence was roughly equal and the degrees cost about the same). But they didn't, and their outcomes weren't equal. That's not necessarily a bad thing, and one choice wasn't necessarily 'suboptimal' either.
And how about a few others?
Someone going to one city over another is a choice, and the consequences could be very different based on that choice. That doesn't mean there necesarily wasn't equality of opportunity.
Same for things like what job you choose (assuming they're 'equal' on a technical level), who your friends are, what hobbies you have, etc. You can probably think of thousands of examples where people are making choices that lead to inequal outcomes, simply because either the market demand is different, attitudes are different or random factors in the world are different between them.
Choices don't necessarily have to be 'suboptimal' to have different outcomes.