One reason might be request limits. OpenAI's ChatGPT Plus w/Codex ($20/month) provides a worst-case 5-hour-request-limit of 15 for GPT-5.5, 20 for GPT-5.4, 60 for GPT-5.4-Mini. Whereas Z.ai Lite ($18/month) provides a worst-case of ~80 for GLM 5.2 (off-peak; on-peak is 2am-6am New York time). So Z.ai can provide higher limits for a cheaper price. (https://codeberg.org/mutablecc/calculate-ai-cost/src/branch/...)