Because the OP took a course that required learning to use tables, not a dive computer. The PADI course does provide RDP tables for two mixes, I think they are 32% and 36%. If that plus an MOD chart were the only practical skills to learn it would still be pretty simple and straightforward.
Where it gets messy is learning to use the DSAT Oxygen Exposure Tables, which allow you to:
- Dive any mix between 30% and 40% without a dedicated table, and to
- Track Oxygen Exposure for multiple dives within a 24 hour period.