Why does an overwatered plant look exactly like a thirsty one?

Because both end in the same place: the roots cannot take up water. The plant wilts for the same immediate reason in both cases, which is why the symptom is genuinely ambiguous and why so many struggling plants get watered to death.

What wilting actually is. Plant cells are held rigid by turgor pressure — water pushing outward against the cell wall. When water leaving the leaves through transpiration exceeds what the roots supply, turgor falls and the plant goes limp. Wilting reports a supply failure, not a cause.

Underwatering is the obvious version: no water in the soil to take up.

Overwatering produces the same outcome by a different route. Waterlogged soil has no air in the pore spaces. Roots need oxygen to respire and to do the active work of absorbing water and nutrients. Deprived of it they stop functioning, then begin to die and rot. A plant sitting in wet soil with dead roots is physiologically drought-stricken, which is the part that seems paradoxical.

How to tell them apart:

Feel the soil, several centimetres down. The surface is a poor guide — it dries first while the bottom of the pot stays saturated. This single check resolves most cases.

Lift the pot. Weight is a far better indicator than appearance, and learning what a well-watered pot weighs is the most useful habit in houseplant care.

Look at the leaves. Underwatered leaves go dry, crisp and papery, usually from the tips and margins. Overwatered leaves go soft, limp and yellow, sometimes with brown patches surrounded by yellow, and lower leaves drop.

Smell the soil. Rot has a distinctive sour smell.

Check the roots. Healthy roots are firm and pale; rotting roots are brown, soft, and slough off when touched.

Overwatering is the more common killer, and it is frequently a drainage problem rather than a volume problem — a pot with no hole, a saucer left full, or dense compost. Watering thoroughly and then letting it dry is better than frequent small amounts.