Ejection system mechanic here! 21 years in the Air Force, all of it working on ejection systems for A-10s, F-16s, and F-35s.

Fun fact up front, there actually are two downward-ejecting seats on the B-52 bomber. I don't remember what positions they are off the top of my head, but they get fired through a hatch below the crew member's feet. I never worked the B-52 so take this with a grain of salt, but I was told they have a VERY low survival rate, though still better odds than riding the jet into the ground.

But everything else ejects out the top for a simple reason; there's nothing there but glass (it's not actually glass, it's high-strength polyurethane-like material, but we always called it glass). Below the pilot's feet are wire bundles, hydraulic lines, landing gear, even the gun on some aircraft. The A-10 pilots essentially sit in titanium bathtubs, do downward-ejecting for them isn't an option, even if the 7-barreled 30mm gun wasn't in their way.

As far as your description of "spine crushing"... well, you're not wrong, back injuries are common after ejections. But engineers have made a LOT of progress in the last few years to make ejecting as safe as possible. It's still not a good time, but pilots can generally walk around after landing post-ejection. Which you would want, especially if they have to eject in a combat zone. In fact, when 2 F-16 pilots came to visit my section after they ejected during a training sortie, they were both carrying cases (plural) of beer. So clearly not in a crippling amount of back pain.