But the standing rocket can only get re-used once every couple of weeks, at best, because it's standing
, and you have to restack it because an SSTO isn't viable. An airliner can turn around about as fast as you can fuel it.
A re-usable vertical rocket will probably never land on
the launch pad next to a tower because of the collision hazard, given that an empty stage responds about 10 times more to wind gusts than a fueled stage. So the vertical stage will most likely land in the middle of big concrete slab the size of a mall parking lot. Then you have to lift it up and secure it a transport vehicle, which will probably take a day, and then roll it back to the VAB, which will take half a day.
When the second stage lands and you have to repeat the process, then lift the second stage to restack the rocket. Then you have to transport the whole thing back to the pad. In the case of a Saturn V, even empty it weighs over a quarter million pounds and is 350+ feet tall. Transporting it is like moving a 35 story skyscraper, and it's subject to wind loads, so the crawler has to weigh 6 million pounds
and keep the rocket vertical to within a sixth of a degree.
The fastest the vertical Saturn V was assembled and launched was about two months. The fastest the highly-resuable Space Shuttle was turned around was 54 days. Beating that with a tall
vertical rocket is going to be hard, because tall things are inherently hard to work on, hard to move, and hard to assemble. On its side, a Saturn V is trivial, no bigger than a small WW-II destroyer.
The point is that if you want a cheap system
, you can't do it vertically because it adds too much cost to every mission
. It takes a crew of 30 just to drive the crawler, and then you have all the pad support. You're adding hundreds of personnel to each launch, every time, and limiting the flight rate to once every few weeks, more likely about a dozen launches a year. In that case you've addd about a million or two dollars to the cost of the launch, which is five or ten times more than the fuel cost. It's not important when the system you operate is extremely expensive, but it becomes extremely important if you got reliable and cheap enough to where even fuel was a significant part of the bottom line.
Going up horizontally adds complexity, but we often add complexity to lower operating cots. All jump jets add enormous complexity to rotate the thrust through 90 degrees because it's vastly better than trying to tail-sit, and line maintenance on a tale-sitter would be a nightmare requiring lifts and access ladders.
And you already have engines near the front of the rocket, used to propel the second stage. Currently they're just dead-weight during the launch. Cross-feed them from the main tank and you gain the lift-off advantages of a strap-on or piggy-back configuration with the low drag of a stacked configuration later in the flight.
And of course you can always build a re-usable stage 0 whose only purpose is the first minute of flight, handling the entirety of the horizontal load and the lift-off thrust requirements so that you not only don't suffer a performance loss, you get a performance gain, while completely avoiding the needs for a VAB, cranes, lifts, crawlers, and towers. All you'd need is a pre-fab commercial building or conventional aircraft hanger, a truck driver, and a strip of concrete.