Earth's gravity is too strong and human energy exploitation isn't large enough to allow for such reusable rockets. The rockets we humans fly look massive and shame to discard, but they're more like liquid fueled balloons than proper spaceships for their scales.

e.g., the Al-Li metal layer of Shuttle ET was 0.1" thick at thinnest points yet those ET were 150ft+ tall. iPhone back covers are thicker than that. Rocketry gears are infuriatingly flimsy relative to their size, but they have to be because that's what it currently takes to fly to space.

If we could build ships on the Moon from Lunar rocks out of a grinder, or if we could build an all-fusion spaceships, we can (relatively)easily have 2m thick radiation shielding, or 8km wide hulls, or anything we want.

But we're not doing it, but are stuck with lox-fuel chemical propulsion, so we can only make them so durable.