It’s about lift generation and gravity. Planes stay aloft because of the lift generated. So plane takes off near horizontal, with engines creating thrust in a near horizontal vector. The shape of the wing, combined with the near horizontal thrust vector creates lift, which is perpendicular to the thrust vector, and is what exceeds the pull of gravity, so you climb, while also moving forward. Depending on how you angle the wing, you change that lift force/vector so you can climb, fly level or decend.

If you angle a conventional plane vertically, it will still generate “lift” but that lift will be angled perpendicular to to gravity force. In reality, the plane “stalls” before vertical—this stalling means the wind angle has gone beyond where it can generate enough lift to keep the plane level or climbing. Simply put, most aircraft engines are completely insufficient to escape gravity on their own, they’re using a mechanical advantage via wing generated lift to stay up.

Space rockets use an immense amount of force to escape the atmosphere, they’re basically using a direct vector force to cancel out and exceed gravity, as well as friction. This requires fairly mind boggling amounts of fuel (energy) to do, which is why pounds of cargo capacity are extremely limited.

A VTOL aircraft that has thrust vectoring, can aim thrust down vertically to rise off the ground vertically for a period of time, and then rotate the thrust to the rear to enter into standard lift based flight. I don’t know this exactly, but I suspect the vertical portion of the VTOL sequence is much more energy intensive than the horizontal portion.

Helicopters are neat because they generate vertical lift, but that rotor plane is also capable of behaving like a wing, allowing them to mimic some aspects of fixed wing flight. For instance, if your engine does, you can use autorotation (basically as you fall, it spins the rotors, and you get wing lift so you can “glide” in to land safely).That said, helicopters are less efficient than a fixed wing, which is why if you fly across the country you’re in a large plane, not a helicopter.

I’m sure there are scientific details I’m missing here, but that’s my layman’s understanding of why you can’t point a standard aircraft vertically and fly straight up.