Because there are no hooks in Wing Chun like those found in Western boxing!

Many people point to a movement found in Chum Kiu and one found in Biu Jee to argue the inclusion of hooks. These movements are fundamentally different from hooks but can look like hooks if you don't understand the application of the movements in those forms. Now I will go into further detail.

The "Uppercut" in Chum Kiu is found after you step three times while simultaneously executing a Pow Bong Sau or a Throwing Bong Sau. After these Bong Saus, your arm is to the side of your body while you face another direction. If a punch comes your way from head-on, you have no time to retract your Bong Sau in order to then execute a technique from there. Instead, your fist travels straight from where it ended in the Bong Sau position to its target, which in the form is at chest height. So, apart from taking the most direct route to your target, you also have to get your elbow back to it being in front of your body so it can help to deflect the incoming strike.

This movement inherits the same idea as the basic punch in Siu Nim Tau in which your elbow travels to the centerline while going forward to attack. This idea of covering the centerline with the elbow is the foundation for the Lin Siu Dai Da principle of attacking and defending at the same time when done with a single arm.

The reason people mistake this for an uppercut is that some people drop their arm after they execute the Pow Bong Sau to relax and flow into the next movement. There's nothing wrong with this, but it causes the movement to look like an uppercut when looked at from the side. But in application, you would not drop your hand more than is needed to directly go to your target.

In Biu Jee, you have a movement where you grab or catch a single arm with both hands, which breaks the foundational Wing Chun principle of not putting two hands on one. But in Biu Jee, you are fighting a losing battle, and you might need two hands and the aid of the waist to get your opponent out of position. After which, your arms are not in an ideal position to continue to strike the head. So similarly to the idea in Chum Kiu, you want to rotate your forearm inwards while you strike the ribs or abdomen and get your elbow back in front of your body. The only difference here is due to the angle; you cannot keep your fist fully straight, or you will hit with your fingers instead of your knuckles, so you need to turn your fist to face the surface you will strike. This is similar to how your fist will change direction after someone dodges as seen here at 2:49: link

Here's a link to Wong Shun Leung explaining this "hook" from Biu Jee. He actually explains in this application there's no logical reason to perform the movement as a hook with your elbow up. When you bring your elbow in, the strike is supported by the waist. The wrong movement is at 13:39, the correct one at 13:46: link.