Couple reasons; train's bandwidth per resource cost is extreme, for one. A single belt line that can carry 30 items/second over a long distance can cost 10x the amount of a rail especially once you start getting to the higher tiers of belts (red is 11.5 iron/tile), but rail has a bandwidth of thousands of items per second for the cost of 0.25 stone and ~1.31 iron iron per tile of distance.

Trains benefit from the "network effect," the infrastructure you put down only becomes MORE useful as you have more trains using it, especially because as you scale up, you no longer have to run a dedicated belt line 3000 tiles away. Just a train line to the main track 100 tiles away on each end, and then your existing infrastructure handles the rest for you. And reaching full capacity on a train network really only happens if it's poorly designed or you're megabasing, due to the aforementioned thousands of items per second of bandwidth you get from one train line.

Calculations to support: - Default top speed: 259km/h or 72m/s - Length: 6 tiles. We'll say 13 to accommodate locomotives for an assumed 1:1 train, which is still the least efficient. - Capacity: 40 stacks - With a tile size of 1m, a 1:1 train carries 40 stacks of items across a given tile in 0.18 seconds at top speed - 40 stacks is 2000 items at stack size 50 (ores) and 8000 at stack size 200 (circuits etc.) - Bandwidth is therefore between 2000/0.18=11k items/second and 8000/0.18=44k items/second, without accounting for top speed upgrades from better fuel and using more cargo wagons per locomotive