Because flex just references the layout engine, and div intentionally has no semantic meaning. Fwiw I don't think InlineStack is a particularly good name either

Because flex just references the layout engine, and div intentionally has no semantic meaning. Fwiw I don't think InlineStack is a particularly good name either, but the criticism of this needs more justification than just "it's not how we did things in the past". Semantics evolve over time. This is true of natural language and coding practice.