Say for example a 125cc motorcycle with 6 gears has a top speed of about 75mph, why can’t they make it with 7 gears and surely that’d increase the top speed?
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Think about a bicycle. In a higher gear you can achieve a higher top speed given the same RPM of the pedals, but you have to push them harder. Make the gearing [too big](https://cyclingtips.com/wp-content/uploads/2020/12/bigRing.jpg) and you aren’t able to push hard enough to get up to speed. As your speed increases so does the amount of drag, if there were no drag you could push that really high gear up to ridiculous speeds given enough space. The record for the fastest bicycle was set at 184mph using a [special vehicle faring](https://images.wsj.net/im-26794?width=1280&size=1) to block the wind. Because of all this, the best gearing for typical bikes is one that keeps the pedals at an optimal RPM for efficient power to balance against drag. The same is true in a motor vehicle, it’s a balance of power and efficiency.
Edit: you said motorbike and I said car at the end, whoops.
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