Here's why:

* Kinks are for specific applications: Kinks are found in clinical thermometers to prevent the mercury level from falling back down when the thermometer is removed from the body. This allows the temperature to be read after the thermometer is taken out.

* Lab thermometers are meant for continuous readings: Lab thermometers are used to continuously monitor temperature changes during experiments. A kink would hinder this process by preventing the mercury level from moving freely with the temperature.

* Accuracy: A kink could introduce inaccuracies in the reading as the mercury level would be artificially held at a specific point.

Therefore, lab thermometers are designed with smooth, uninterrupted capillary tubes to ensure the most accurate and responsive temperature readings.