Because He nuclei are not the same as He atoms since they are not electrically neutral. Also, they generally have higher energy after being released from the la

Because He nuclei are not the same as He atoms since they are not electrically neutral. Also, they generally have higher energy after being released from the larger Uranium or other atomic nucleus which causes them to behave differently from an He nucleus that wasn't released as an alpha particle. This all is important in terms of tracking what happens to subsequent decay reactions.