I agree, we don't "use the longest leg motors you could find". But you are not thinking of the lever correctly. The force is applied to the water at the prop, here the force is the output of the motor and does not change relative to the water. Speed comes from the force applied to the water which does not change regardless of the length of the mid section or transom height.

What I say changes is the force applied to the boat - at the contact point closest to the prop. The force is not being applied to the boat at the prop; the force is being applied to the boat closer to the pivot than the prop and is amplified by the difference in length between the prop and where the mid section contacts the boat.

I am not saying there is a lever effect against the water, there is a lever effect against the transom structure (which is the subject of this thread).

This is the reason thrust blocks are used and why motor/transom brackets are not shorter (just an inch or two from the tilt axis total length). If there was no thrust block and a shorter transom bracket, the transom bracket would twist the top of the transom right off the boat because of the distance between where the force is applied to the water (the prop), and where the boat receives it (the thrust block or bottom of midsection/transom bracket).

Think of putting a pipe wrench on the transom of a boat and pushing with a 100 pound weight. If the wrench is supported against the transom half way from the top to the bottom and half the length of the wrench handle you probably can't break the transom. Move the support closer to the wrench jaws and you become more likely to break something off the boat. Move the support more to the bottom of the boat and closer to the far end of the wrench and there is less multiplied force on the boat because the length between the 100 pound force and the boat is less.