For 4x100 m and 4x200 m races, the respective lanes for teams are fixed. It was seen during the heats of 4x400 m in Tokyo 2020 olympics that in few cases, there was lot of confusion during change of baton. Few racers had to run a little wider to avoid jostling and that costs time. It can easily be resolved if the lanes are fixed for each team. Why has it not been considered?
Your question is based on a false premise. This has of course been considered, and rejected for a very good reason: the outer lanes are longer than the inner lanes.
For the 4x100m, this isn't an issue - the difference isn't that big when running one lap of the track, and the same distance is used for a regular 400m race anyway.
For the 4x200m, the stagger has to be doubled, and you're already in a state when the outer lane has to pretty much start at the end of the first bend in order to equalise the distances. This already makes it hard for spectators to get a good idea of which team is actually leading at any point due to the large spatial distances between teams when they handover.
If you now tried the same for the 4x400m, you'd have to double the stagger again and you'd end up with the team in the outer lane starting somewhere around about the start of the second bend (i.e. what is the end of the back straight for the team in the inner lane). This would be getting silly, so instead teams break after the first 500m.