There are two main components to standing that could be the issue.
The first is the fact that taking weight off the saddle means it has to go somewhere. In a hard sprint, it’s going to the pedals. A lazy climb, it’s going onto your hands. This is *significantly* harder on your body. Some people have the cardio and the muscular endurance to put down respectable power all day, but lack the torque necessary to stand up. Or, they may lack the upper body strength to support a big chunk of their body weight on their hands.
The second is the handling aspect. Putting more weight on the bars changes how your bike handles. It can also be very off-putting to people who are afraid of flying OTB. Then, of course, you have the rocking aspect. Unless you want to get a ridiculous isometric arm and back workout (most casual cyclists do not have the strength to do this), the bike needs to rock at least a little. Enough to counteract the pedal torque you are putting down.
Besides good instruction, some solid calisthenic or even weightlifting exercises can make a world of difference when it comes to command of the bike.