Why are shower drains 2", but tub drains 1.5"?
Tubs are big and intend to hold the water; drainage speed is not really important. Showers on the other hand have a low curb and can’t hold much water, so it is important that there is plenty of drain availability to prevent overfilling and flooding.

Some tubs do have 2” wastes, but I think smaller drains are more common as it reduces the possibility of completely using downstream pipes via multiple upstream fixtures draining simultaneously.