It's because it would be hard to "perform" if the other party started laughing when you got undressed !

Common routes, practiced navigators, and sheer weight of numbers of vessels.

"how is interstellar economy even possible?" The answer is "it's never was", for one, cause using word "Economy" on a feudal, tithe and obligation-based galactic country of a million planets with 0 administrative cohesiveness is pointless. Luetin09 Explains this in his video on Hives, delves into "Economy" of Imperium: kzfaq.info/get/bejne/ftF6o6mnr7fUZIE.html For another, 40k Imperium is essentially about "Throwing enough sh..t at a wall until some of it is gonna stick". It's not about sending enough supplies to Terra, it's about Sending an impossible to even roughly estimate overkill enough of supplies to Terra in hope that at least 20% of that will ever reach it. Terra is the most important spot for Humanity, so Imperium will literally sacrifice not fleets or armadas, but clouds of millions of ships just to buy it another day. Terra doesn't wonder if they'll arrive, cause they MUST and they WILL, it's part of this tithe system, each world, each colony, each spot blemished by human activity must by feudal obligation produce and send some % of it's yields to Earth. To fail to do so is Heresy, to refuse to do so is Exterminatus-lvl Heresy. Imperium of Man in 40k is not ruled by money, or High Lords of Terra, they are ruled by the Holy mandate to preserve the cradle rock and the resting place of their God.