That depends on the car brand. Mercedes' don't require engine rebuilds because the engines outlast the body they are in. Most cheaper brands don't require engine rebuilds because it isn't economical to do so, by the time the car clocks over 250K its book value is way higher than an engine rebuild.

But for classic sports cars they're fairly normal, those engines were not made to last forever, tend to be fairly high power for their displacement and the book value of the cars is high enough that rebuilding an engine can make sense.