They are a nicer product to use, by people who are relatively well off and will use them repeatedly throughout the day for a decade. $50 (basic stethoscope) vs $200 (premium stethoscope) is simply not that expensive for a doctor or a family buying a graduation gift, even in poorer countries, given the lifetime of the device (easily 10 years of daily use).
Within that space you get things like soft-seal eartips , where as most cheaper models came with hard rubber, a stiffer spring that holds its shape better, and great acoustics (though most users will know what to listen for by the time they pick one up). You also get less tube noise from movement, less rubbing and scratching under the head, and longer listening time without having to fiddle.
The diaphragm on expensive stethoscopes is more complex, often "floating" — not under tension at rest, but when you press down, a ring tightens it. This gives you both bell (low frequency) and diaphragm (high frequency) response from a single side, so you modulate pressure rather than rotating the head.
Why do expensive stethoscopes fail?
1. You lose them.
2. Wear and tear many parts are replaceable, but repairing the whole head often isn't worth it. Rotating heads can become a failure point, growing lax over time, with grit accelerating the damage.
3. Neck oils degrade the PVC tubing.
4. Alcohol/cleaning wipes also degrade the PVC tubing.
Surprisingly, Littmann hasn't released a dedicated long-term care wipe for occasional use.