Championship athletics requires two tracks, not one. The warmup facility beside the stadium is a competition requirement rather than a convenience.
The competition track is unavailable
During a session the main track is running events continuously. There is no window in which dozens of athletes could warm up on it.
Even between sessions it is protected. Surface wear, marking integrity and the need for officials to reset equipment all argue against opening it for preparation.
The result is that a stadium without an adjacent warmup track cannot host a championship, whatever its seating capacity.
Warmup demands a full facility
Athletes need the same distances, the same surface, and in the field events the same runways and landing areas as the competition itself.
A sprinter finishing preparation on a different surface arrives at the start line having calibrated to the wrong feel underfoot.
Throwers need real throwing circles, which makes the warmup area a substantial construction project rather than a strip of track.
The transfer is timed and escorted
Athletes move from the warmup area to the call room and then to the stadium on a schedule, usually escorted, along a route closed to the public.
The length of that route is a real planning input. A long walk means athletes leave the warmup track earlier and cool down more before competing.
Host cities that place the warmup facility across a road rather than adjacent create a problem that no amount of scheduling fully solves.
Support services live there too
Medical staff, physiotherapy areas and team tents cluster at the warmup track, because that is where athletes spend the hours before competing.
Coaches also work there, since access to athletes ends at the call room. The warmup track is the last place instruction can be given.
For a spectator with access, it is often more interesting than the stadium: it is where preparation is visible and where athletes behave normally.
What American venues face
Most large American stadiums were not built with a second track alongside, and university facilities are frequently the only sites where the pairing exists.
That constraint shapes which cities can bid for major meets, sometimes more than seating capacity or transport does.
Temporary warmup tracks can be installed, but they are expensive and the surface match is difficult, which is why permanent pairings remain strongly preferred.

