I’m not great at physics and have no knowledge of aeronautics, so this whole chain of reasoning might be wrong.
A plane stays in the air because air is moving over the wings, which generates lift. However, that air is moving because the engine is moving the plane forward. There is no other source of energy. Therefore, some of the engine’s energy is going into keeping the plane in the air, and some is going into accelerating it forwards, or keeping it at the same speed (fighting air resistance).
Therefore, if the plane points straight up, the engine should be able to support it hovering in the air. If it didn’t have enough power to fight gravity when pointing straight up, it wouldn’t have enough power to fight gravity when moving horizontally, either.
(Okay, some older engines only worked in certain orientations, but I don’t think that’s a problem for jet aircraft, or any aircraft built after WWII.)
So why can only certain planes fly vertically?
Here is an easier example.
Think of a truck that sits on a street that is going slightly uphill. There is no way that you and your buddy could lift that truck straight up in the air. But you can relativily easy push it uphill. In the end the truck is going up.
Now, what’s the physics behind that and how does it relate to planes? Well, you don’t have to lift the entire car when pushing. Most of the force from gravity is resisted by the ground. You only have to push against the much smaller horizontal component that tries to push the car downhill.
With planes you basically just replace “ground” by “lift”. So instead of tires pressing against the ground, you have wings pressing against the air. And a jet engine instead of two guys pushing.
Basically a plane’s engines are pushing the plane up a hill made of air.
Beautifully put … an ELI5 that my five year old like brain can understand