Because the old glass mercury thermometers have been replaced with digital. The glass had to stay in place for minutes at a time and it prevented the thermometer from disappearing. Digital are almost instant so no need to leave in place but there are still those vets that attach a string for a just in case.
Today new and improved means a computer reads and registers the temperature for you, you don't need to do anything but push a button.
Mercury thermometers are obsolete to buy but many still have them.
They work and work well but they take a few minutes of time to read correctly hence the need for a string and mine had a clothespin to attach to the tail so I did not have to stand guard holding the string.
Today, 5 - 8 seconds and you have your information just is faster and no guessing of temperature as digital is also exacting in degrees.
Yes, I have a digital one and it also has a string, just in case cause my fingers don't enjoy being in the butt to that degree actually. 🐴 ...