Toasters have numbers on them instead of time because those numbers represent the level of toasting or browning that you want for your bread. Each number corresponds to a specific duration of toasting time and, therefore, the level of browning you desire. These numbers are usually used to adjust the heating intensity of the toaster's heating elements.
The reason for using numbered settings rather than specific time durations is that the toasting time required can vary depending on several factors, including the type of bread, its thickness, and the starting temperature. Different types of bread (white, whole wheat, bagels, etc.) may require different toasting levels, and individual preferences for toast darkness may vary as well.
Using numbered settings allows for more flexibility and ease of use. It simplifies the process for the user, as they can easily choose their desired level of toasting without having to calculate exact time durations each time they use the toaster. For example, if you prefer a lightly toasted bread, you might select a lower number (e.g., 2), while someone who prefers a darker toast might choose a higher number (e.g., 4 or 5).
In contrast, some modern toasters do come with digital displays and allow you to set specific toasting times. However, the numbered settings on traditional toasters have been a standard and convenient way to control the toasting process for many years.