This is likely because when items are activated by a Dispenser or Dropper the Sensor can detect the item hitting the ground, lighting a fire, or placing liquids, however special events such as a boat, minecart, or armor stand entity being created are uniquely ignored. Boats and minecarts, unlike other non-mob entities (item frame, end crystal, painting, and armor stand), do not activate the Sensor when placed by hand.
Relates to:
MC-114111
MC-207258 Refers to driving minecarts rather than placing them.
MC-93281 Indicates placing by hand is intentional, this ticket refers to dispenser success and not to block/entity sounds.
MC-148455
MC-94060