Airplanes can either turn speed into altitude, power into altitude, or a combination of the two. That’s pretty easy to understand and a lot of the current answers talk about power-to-weight ratio. The Blues need to burn down some fuel before they reach better than 1-to-1 and have the ability to accelerate straight up; they also have to have proper conditions (lower altitude, certain speeds or better, etc.).
Fleet airplanes have even more challenges considering the stores that they ordinarily carry.
When they are going straight up, they are still producing lift and the pilot still needs to fly the airplane. In fact, the lift from the wings (because air is still going over them) is trying to push the airplane onto its back because the gravity isn’t countering the lift (gravity is being countered by the engines).
The Hornet has a danger zone (key the music) once it’s a little bit on its back, around 110º from the horizontal. You want to stay away from that because the flight control computers are working to get you out of there, but there may not be enough control authority (not enough wind over the tail) and you just get stuck there until the ground helps you out. So, you’re actually pushing forward on the stick, which may seem counter to what’s going on.
Now, if you’re talking about the Blues flying really slowly, parallel to the ground with the nose high in the air, the shape of the wing and the strakes along the fuselage in front of the wing are designed to allow excellent slow-flight capability (along with the canted vertical stabilizers).
You need a lot of power there, too, though. In fact, they call that the “back side of the power curve”, where, to get slower, you need to add power to remain in level flight.
Fortunately, the Hornet can sometimes just plug in maximum power and get out of that regime, which otherwise can be very dangerous for lower-powered aircraft. So, in answer, the lift from the wings is working with the thrust from the engines (that portion that is countering gravity) to keep the dude up.
