The main reason why the images that are sent through Whatsapp are already low quality is the need for people to send the images immediately. If the images need to be sent immediately, the images cannot be of high quality. Otherwise, it would take longer. You can change the settings of the application so that you can send images that are still high in quality.
Whatsapp will also allow you to take photos through the app before sending. When you do this, there will be no visible change in the quality of the images. There are always different applications aside from Whatsapp that will also lower the quality of the images that are sent to other people.
Images get compressed on whatsapp to enable very fast sharing. For instance, an image whose dimensions are 3 MB will be reduced to under 70 KB.
You can send an image that isn't compressed if you don't mind waiting aeons while it is transferred. Most people are much more impatient. They take a pic and want to share it immediately. That's why compression was developed, to meet this common requirement.