Why can't you fly a plane to space?
Turns out, you couldn't even make it halfway. Part of the problem is Earth's gravity. You need to escape it in order to reach space.
This requires a minimum speed of mach 33 (25,000 mph). The current world record for fastest plane is only mach 6.7 (5,140 mph). The other problem is the atmosphere.
As you fly higher, the atmosphere grows thinner. This creates two serious issues:
Issue 1: Fewer air molecules means it's harder for the plane to stay airborne.
Issue 2: Less oxygen means less combustible fuel to power the engine.
So how high can planes actually fly? Commercial airliners generally don't surpass 45,000 feet. But some pilots have pushed their planes to extreme limits.
In 1977, Alexandr Fedotov flew to 123,532 feet. This is the highest any ground-launched plane has reached. Yet, Fedotov only made it about 1/3 the way to space.
Another plane, SpaceShipOne reached 367,500 feet in 2004. It had rocket engines and was flown to an initial 43,500 feet before launch.
Looks like its best to leave spaceflight to the real rocket ships.