The issue is that a drainage tube is just an ordinary tube, not some magical item with therapeutic effects.
To make it work, it depends on the doctor's skill to ensure it functions maximally within its effective range.
What is the effective range for a drainage tube? We can liken the drainage tube to a canal, with the source of water (pus) ideally encased like a lake, tightly surrounded on all sides, so the water (pus) inside doesn't run everywhere. If the pus flows everywhere, a tube-like canal can't exert enough attraction to draw all the wandering pus here to drain out, making the effect inevitably compromised.
