Because resolution changes what the model costs to run. A 480p draft and a 4K master are very different amounts of compute, so each second of video is priced at the resolution you actually picked — the exact number is on the Generate button before you press it.
The workflow that saves money: draft cheap, master once. Run your attempts at 480p, find the take you want, then re-render the winner at 1080p or 4K. Misses at draft resolution cost you nothing when you kill them — and the drafts you kill don't gamble away a 4K-sized hold.
Example (Seedance 2.5): 8 credits/second at 480p, 19.2/s at 720p, 25.6/s at 1080p. An 8-second draft holds 64 Credits; the same take mastered at 1080p holds 205.