Learn Russian: Почему ты тащишь эту тяжёлую тележку?

Because the action is happening right now, in one specific direction. The speaker sees the person currently pulling the cart. If the person dragged carts around as a regular habit, or was dragging it back and forth across a room, you would use the multidirectional verb таскать (таскаешь).

Since both are transitive verbs of motion, how does тащить differ from нести (to carry) in this context?

Нести is used when you are carrying something off the ground, supporting its weight in your hands or arms. Тащить specifically implies pulling or dragging something along a surface, usually with significant physical effort. The adjective тяжёлую (heavy) emphasizes why the cart is being dragged rather than easily carried.

Is тащишь imperfective or perfective?

It is imperfective, describing an ongoing action in the present tense. Remember that all basic, unprefixed verbs of motion (like тащить, таскать, идти, and ходить) are imperfective by default. To make it perfective (meaning the action of dragging is completed), you would add a prefix, forming verbs like притащить (to drag to a destination).

Can тащить be used metaphorically, or is it only for physical objects like a тележка?