In custom houses they actually can and often do pre-plan those things. In practice though plumbers know what they are doing without needing to look at the prints and it isn't important enough to make the plumbers put the pipes where the plan puts them. The architect knows this and rarely puts enough effort into those parts of the plan to get them perfect, just enough to be sure that where space is tight everything can fit.
No plan is perfect the first time. For pre-fab you do a few prototypes and find out and fix what doesn't work, then make the jigs so everything works. For custom work the trademen are smart enough to get it working without extensive pre-planning.
The above only applies to small scale single family houses. As you get into larger buildings: correct planning becomes very important. Eventually things must planed correctly in the design phase or the whole will not work.
No plan is perfect the first time. For pre-fab you do a few prototypes and find out and fix what doesn't work, then make the jigs so everything works. For custom work the trademen are smart enough to get it working without extensive pre-planning.
The above only applies to small scale single family houses. As you get into larger buildings: correct planning becomes very important. Eventually things must planed correctly in the design phase or the whole will not work.