This question seems to hinge on a fundamental misunderstanding about space, that is, to be fair, extremely common among the general public. It's the idea that space has no gravity, so things in space are weightless.
"But wait!" you say. "I've seen videos of astronauts in space, and they sure seem weightless to me." And you'd be right, they do seem weightless... but they're not. They're in a state known as "freefall".
Now, "freefall" as a technical term means that the object doesn't have any forces other than gravity acting on it. So, if you shot a gun straight up, then it would be in freefall from the moment it left the barrel to the moment it hit the ground (ignoring air resistance), even though it was only "falling" in the colloquial sense for half of that time. Things in freefall appear weightless to other things that are also in freefall, so that's how astronauts can float around a spaceship. In fact, it's been proven that you can't tell the difference between actual weightlessness and freefall in a uniform* gravitational field.
So, if you try a direct ascent from the Earth, you'll be fighting gravity the entire time. If you tilt over and thrust horizontally, however, you can build up your speed without having to fight gravity, which makes intermediate parking orbits much more efficient than direct ascents**, even before you take into account things like the Oberth Effect.
You mentioned that they would have had to achieve about 28,000 km/h to enter a stable orbit, implying that a direct ascent would be slower. But that's not true. Yes they had to go that fast to orbit, but then they had to accelerate even more (by about another 6,000 km/h) to go fast enough to avoid falling back to Earth, and even then, they were only going in the neighborhood of 3,000 - 5,000 kph when they reached the point where the Moon's gravity became greater than the Earth's and they started accelerating again. That's the same speed that a direct ascent would have to achieve, albeit without the aforementioned efficiency boost.
Please note that I'm not a physicist, so I'm probably not explaining this very well. I'm using "fight gravity" in a colloquial sense. Basically, gravity is always pulling you down, so any fuel you use to go up is opposed by gravity. As an extreme case, picture a rocket weighing a total of 100kg, with an engine that produces 981N of thrust. If pointed straight up, the thrust will be perfectly balanced by its weight (ignoring mass reduction due to fuel burn), so it will waste all its fuel hovering in one place going absolutely nowhere. Turn it on its side, however, and suddenly it's accelerating faster than a Ferrari, going from 0 to 100km/h (60mph) in 2.8 seconds.
The same thing happens in space. Any vertical thrust (e.g. radial out (away from the planet) or radial in (toward the planet)) will have to overcome both the spacecraft's inertia and gravity, while thrusting horizontally (prograde (forward), retrograde (backward), normal (left), or antinormal (right)) only has to contend with the spacecraft's inertia, and so is more efficient. You can see this during a launch: rockets have to thrust radially for a short time to get them above the thickest part of the atmosphere, but then they pitch over to a horizontal attitude as soon as they reasonably can in order to avoid burning more fuel then they have to. Low Earth orbits are on the order of about 7.5km/s, but spacecraft launched from the surface typically have a delta-v capability of 8 to 8.5 km/s, that extra delta-v being lost to gravity during the short vertical ascent phase.
During the planning stages of the Apollo program, direct ascent was considered as one possible launch strategy. It's benefit was that it would be a much simpler plan than worrying about an orbit, and they were in a major time crunch. One of the reasons it was shelved is that they didn't have a facility big enough to build the enormous rocket that such a plan would require.
*Note that Earth's gravitational field isn't precisely uniform. Thus, astronauts do feel a very tiny bit of gravity that changes in strength and direction depending on where they are in the ship. That's why official NASA literature refers to "microgravity" rather than "zero gravity".
** Okay, technically you don't have to actually orbit to get the efficiency boost. It's the horizontal thrust that's important here. So you can thrust horizontally from suborbital speed all the way up to escape velocity, without ever achieving an official "orbit". But stopping partway to check that everything's working and that you're properly aligned for the next engine burn is just good engineering, as @jamesqf mentioned.