It comes down to the numbers. Falcon 9's first stage accelerates at about 12.5m/s^2. That's about 1.25g's. It does that for 161 seconds, and first stage separation happens at an altitude of 100km, at Mach 6. That'd be A LOT of track to build straight up. The troposphere ends at 20km. People have trouble breathing before you get to there.

Pilots try to keep plane flight take-offs below 1g. You could possibly increase the g load but not everyone is conditioned like a fighter pilot. Even supposing you could get up to speed faster, you'd have a lot more drag in the lower atmosphere. You can't really get up to any significant speed until you exit the troposphere.

And that means building at least 20km, straight up.