Belt length is not free. More belts cost resources and more belts take up more land space. This land space could instead have been used for trains or roboports.Same as with any length of it.
Logistic power is an excellent way to compare the opportunity cost of belts vs. bots in terms of land space. Packing a lot of logistic power into a small land space is the secret to making beacon builds work because beacon builds give a limited amount of land space to supply your items. It isn't very good for comparing resource costs because yellow belts and trains are the cheapest way to move items by far. However just because a yellow belt is cheap doesn't mean it's the best way to move items.