While you have a good point about the proprietary middleware, that would be far from inherently unportable - just not yet ported, likely due to low profitability. Something truly unportable would rely on specific hardware quirks, like - and I’m very sorry I can’t find the original source - one old instance of some game that relied on the timing of a spinning drumhead in order to select the correct data or instruction after the completion of one prior; and even then, a port could be made with sufficient understanding of the original system.