A cross-country drive planned purely around the fastest route usually means missing most of what makes a road trip worth doing in the first place - the stops along the way, not just the destination.
A route optimized for speed looks very different from one optimized for scenic stops or specific attractions. Deciding which this trip actually is before mapping anything out avoids constantly re-planning halfway through.
A route that looks reasonable on a map often underestimates how tiring long driving days actually are. Capping daily driving at a comfortable number of hours, rather than the theoretical maximum, keeps the trip enjoyable rather than a physical endurance test.
A rigid day-by-day plan rarely survives an unexpectedly great stop worth lingering at, or a closure requiring a detour. One flexible day built into a longer trip absorbs these without derailing everything that follows.